Full Duplex Bidirectional Transmission on Coaxial Cable in CATV Network

ABSTRACT

Systems and methods for achieving full duplex bidirectional transmission across coaxial cable in a hybrid fiber-coaxial cable TV network. Some preferred systems and method will attenuate reflections propagated within the coaxial cable. Other preferred systems may echo-cancel reflections propagated within the coaxial cable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of priorityto the filing dates of U.S. Provisional Application No. 62/294,369 filedon Feb. 12, 2016, U.S. Provisional Application No. 62/300,763 filed onFeb. 26, 2016, and U.S. Provisional Application No. 62/300,483 filed onFeb. 26, 2016.

BACKGROUND OF THE INVENTION

Cable TV (CATV) systems were initially deployed as video deliverysystems that, in their most basic form received video signals at a cablehead end, processed the signals for transmission, and broadcast them tohomes via a tree-and-branch coaxial cable network. In order to delivermultiple TV channels concurrently, early CATV systems assigned 6 MHzblocks of frequency to each channel and Frequency Division Multiplexed(FDM) the channels onto the coaxial cable RF signals. Electricalamplifiers were inserted along the transmission path to boost thesignal, and splitters and taps were deployed to deliver the signals toindividual homes.

As the reach of the systems increased, the signal distortion andoperational costs of long chains of electrical amplifiers becameproblematic, hence over time larger segments of the coaxial cable in thetree-and-branch transmission network were replaced with fiber opticcables, creating a Hybrid Fiber Coax (HFC) network. The HFC network usesoptical fiber to deliver the RF broadcast content from the head end tothe remaining segments of coaxial cable in the network neighborhoodtransmission network, which in turn delivers it to the subscribers.Optical nodes in the network acted as optical to electrical convertersto provide the fiber-to-coax interfaces.

Over the years, HFC is continually evolving to push fiber deeper in tothe network. Eventually, it will reach the point where it becomes aFiber to the Premise (FTTP) architecture, but this may take decades atan economical pace. FTTP is happening today in new Greenfielddeployments, yet there are significant operational challenges to makethis transformation in existing HFC infrastructure, a.k.a. Brownfields.

Simultaneously, the HFC network is evolving to deliver an increasingamount of content to subscribers, as well as provide data (e.g.,Internet) services at ever-higher speeds. Such data services are IPpacket-based services, but are propagated on the HFC network asadditional frequency blocks that use FDM to share the spectrum alongwith video services. Unlike broadcast video, each IP stream is unique.Thus, the amount of spectrum required for data services is a function ofthe number of data users and the amount of content they are downloading.With the rise of the Internet video, this spectrum is growing at 50%compound annual growth rate and putting significant pressure on theavailable bandwidth. Pressure on the available bandwidth has furtherincreased with the advent of narrowcast video services such asvideo-on-demand (VOD), which changes the broadcast video model as userscan select an individual program to watch and use VCR-like controls tostart, stop, and fast-forward. In this case, as with data service, eachuser requires an individual program stream.

Unlike broadcast video, data services require a two-way connection.Therefore, the cable plant must provide a functional return path, i.e.data communication between the CATV head end and subscribers includes adownstream path that delivers video and data to subscribers, along witha return path that delivers data from the subscribers to the head end.To prevent interference between the upstream and downstream signals whentransmitted over HFC network, separate ranges of bandwidth werededicated to these upstream and downstream signals respectively, suchthat a smaller, low-frequency range of the total transmission spectrum(for the upstream signal) was “split” from a larger, higher frequencyrange (for the downstream signal). As can easily be appreciated, as morevideo content and faster data services are provided via the HFC networkover time, the “split” between the upstream and downstream paths mustchange. Historically, HFC systems have supported several differentsplits, including 42, 55 and 65 MHz splits. The DOCSIS 3.0 standardintroduced a 85 MHz split, but this split not been widely deployed dueto the difficulties of moving legacy services (e.g. STB control channel,FM channels) from existing 54-108 MHz spectrum reserved for downstreamcontent. Moreover, the DOC SIS 3.1 standard further contemplates asignificant increase in upstream spectrum, and associated capacity, withthe option of a 204 MHz upstream split with the corresponding downstreamspectrum starting at 258 MHz. This however exacerbates the difficultiesarising from supporting legacy downstream services in the 54-258 MHzrange.

Rather than migrate to new architectures, such as fiber-to-the-premises(FTTP) where fiber replaces all portions of the CATV network, manyexisting CATV providers have tended to squeeze as much content andservices as possible over the existing CATV architecture. However, thecapacity of the existing HFC architecture is limited, and this solutionwill be adequate for only so long.

What is desired, therefore, are improved methods and systems fortransmitting the breadth of content contemplated by the DOCSIS 3.1standard over an HFC network while simultaneously providing support forlegacy downstream service.

SUMMARY OF THE DISCLOSURE

In a first embodiment, a system may include one or more ONUs togetherdelivering respective upstream and downstream content to each of aplurality of subscribers in a single service group, and from a commonoptical input signal. The one or more ONUs together configure theupstream and downstream content delivered to a first subscriber to havea first split, and configure the upstream and downstream contentdelivered to a second subscriber to have a second split different thanthe first split.

In a second embodiment, a method may comprise delivering content to afirst subscriber using a first coaxial length from an ONU associatedwith the first subscriber, and over a first temporal interval, whiledelivering content to a second subscriber using a second coaxial lengthfrom an ONU associated with the second subscriber, and also over thefirst temporal interval. The method may then deliver at least one ofFTTLA, FTTC, and FTTH to the first subscriber over a second temporalinterval subsequent to the first temporal interval, by replacing atleast a portion of the first coaxial length with a fiber opticconnection. During the second temporal interval, the second subscribercontinues to receive content over the second coaxial length from the ONUassociated with the second subscriber.

In a third embodiment, an ONU may have an input that receives an opticalsignal propagating upstream and downstream CATV content, and at leastone output that delivers the upstream and downstream content to a firstsubscriber in a service group comprising a plurality of subscribers. TheONU may also have a split-setting element that configures the splitbetween the upstream and downstream content to the first subscriber in amanner independent of the split between upstream and downstream contentto at least one other of the plurality of subscribers.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS

For the purpose of illustrating embodiments described below, there areshown in the drawings example constructions of the embodiments; however,the embodiments are not limited to the specific methods andinstrumentalities disclosed.

It is noted that while the accompanying figures serve to illustrateembodiments of concepts that include the claimed invention, and explainvarious principles and advantages of those embodiments, the conceptsdisplayed are not necessary to understand the embodiments of the presentinvention, as the details depicted in the figures would be readilyapparent to those of ordinary skill in the art having the benefit of thedescription herein.

FIG. 1 shows a “Cloonan Curve” and Nielson's Law.

FIG. 2 shows a Network Quality of Experience (NQoE) formula.

FIG. 3 shows three exemplary CATV network traffic engineering scenarios.

FIGS. 4A and 4B show an exemplary 20-year growth window for a MaximumSustained Traffic Rate parameter.

FIG. 5 shows an exemplary network capacity model having 128 subscribersper service group.

FIGS. 6A-6D show exemplary network capacity models having service groupswith subscribers of varying sizes.

FIG. 7 shows example plant upgrade costs associated with a disclosed“Selective Subscriber Shedding” method.

FIGS. 8A and 8B show HFC Upstream and Downstream capacity for selectedsizes of spectrum transmitted by a CATV provider.

FIG. 9 shows an example of HFC spectrum overlapping with HPON spectrum.

FIG. 10 shows a comparison of downstream capacities for various CATVarchitectures.

FIG. 11 shows a Fiber-to-the-Tap architecture.

FIGS. 12-15 show respective steps in an exemplary HPON topologymigration

FIGS. 16-18 show respective alternate exemplary HPON topology migrationexamples.

FIG. 19 shows a set of MER curves for various RF spectrum loads on afirst generation HPON splitter system.

FIG. 20 shows a comparison of HPON upstream capacity between OFDMA andSC-QAM.

FIG. 21 shows HPON Downstream RF Performance.

FIG. 22 shows a comparison of HPON downstream capacity between OFDMA andSC-QAM.

FIG. 23 shows an exemplary EPON burst diagram.

FIG. 24 shows the results of a simulated Kramer analysis.

FIG. 25 shows the control overhead efficiency for 10/1 and 10/10 EPON.

FIGS. 26-28 shows the results for different scenarios of upstreamcoexistence capacity.

FIG. 29 shows an example average transmitter burst size required forupstream traffic load.

FIG. 30 shows an example average transmitter burst size for heavy users.

FIG. 31 shows an economic analysis of HPON.

FIG. 32 shows a chart of the number of fiber trunks needed to servespecified numbers of users in a service area.

FIG. 33 shows relative energy costs for HFC, EPON and HPON systems.

FIG. 34A shows a single-output ONU having a customizable split betweenupstream and downstream signals.

FIG. 34B shows a multi-output ONU having customizable splits betweenupstream and downstream signals for each output.

FIG. 35A shows an exemplary directional coupler capable of use in theONUs of FIGS. 34A-34B.

FIG. 35B shows an exemplary diplex filter capable of use in the ONUs ofFIGS. 34A-34B.

FIG. 35C shows an exemplary hybrid filter capable of use in the ONUs ofFIGS. 34A-34B.

FIG. 36A shows an ONU with an integrated diplexer.

FIG. 36B shows a multi-output ONU having receptacles for plug-indiplexers.

FIGS. 37A and 37B each show an ONU of FIGS. 36A and 36B, respectively,having a PON-pass through element.

FIG. 38 shows an RF amplifier used in a coaxial network having twodiplex filters.

FIG. 39 shows and alternate amplifier that replaces one of the diplexfilters in the amplifier of FIG. 38 with an ONU.

FIG. 40 shows an ONU upstream burst detection system and laser biascontrol.

FIGS. 41A and 41B show respective output spectra for different rise timeof the laser shown in FIG. 40.

FIG. 42 shows the output when the ONU of FIG. 39 turns on in 500 nsfollowing detection of an RF input signal.

FIG. 43 shows an alternate ONU upstream burst detection system and laserbias control.

FIG. 44 shows the improvement in clipping that results from using thebias control of FIG. 43

FIG. 45 shows the output of an ONU with a delayed RF gain control and afaster rise time.

FIG. 46 shows another alternate ONU upstream burst detection system andlaser bias control.

FIG. 47 shows an exemplary multiport ONU connected to user cable modemsvia coaxial cables, and which mitigates reflections.

FIG. 48 shows a CATV architecture capable of implementing the systemsand methods disclosed in the present application.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As noted previously, DOCSIS 3.0 and its 85 MHz split between upstreamand downstream services was not implemented to its full extent due tothe inability to make available the higher quality services contemplatedby that standard while simultaneously supporting legacy devicesconfigured for a lower split between upstream and downstream signals. Asalso noted previously, this problem will be exacerbated by the plannedimplementation of DOCSIS 3.1. with its split where the upstream signalends at 204 MHz and the downstream signal begins at 258 MHz.

Competitive pressures to offer Top-Tier 1 Gigabit (1 G) services havealready induced some cable operators to offer such services over FTTPnetworks. However, integrating an FTTP solution within an existing HFCnetwork requires that all homes on a given coax segment be migrated tothe same new RF frequency upstream split, which creates logisticalproblems with large service group sizes to get every home transitioned.Moreover, this approach is merely a short-term solution that, at currentgrowth rates, might suffice for five to ten years before the growth insubscribers desiring the Top Tier service hits the HFC limits.

Eventually, to fully implement DOCSIS 3.1, all subscribers in the HFCnetwork will need to be migrated to FTTP. The time needed to implementthis transition will likely be significant. From an economicperspective, one analysis shows that this transition will require atleast a 20-40 year window to migrate all HFC subscribers to an FTTPnetwork. Assuming, for example, that an operator transitions 5% of itssubscribers from HFC to FTTP each year, which is an aggressive andexpensive timetable, the full transition would require twenty years.Based on historical spending on plant upgrades, even this scenario isoptimistic.

Given that the transition will take multiple decades, extending theuseful life of HFC to the end of this transition period is of crucialimportance. This will require the adoption of DOCSIS 3.1 technologiesand the use of an intelligent strategy for moving subscribers to FTTP.This specification describes systems and methods that implement a mixedHFC/FTTP architecture where selective Top-Tier subscribers are migratedto FTTP service (a ““Selective Subscriber Shedding” method), whilemaintaining support for legacy equipment, even where the legacyequipment is used by a one or more subscribers in the same service groupas subscriber(s) receiving Top-Tier service. Utilizing the disclosedsystems and methods will extend the life of HFC for decades,facilitating the smooth and economical transition to FTTP.

Specifically, the disclosed systems and methods support an FTTP overlaythat allows a single home—or a subset of homes in a service group(SG)—to be upgraded to a new upstream split, such as 204 MHz, while therest of the service group continues to use a legacy upstream split. Thedisclosed systems and methods will provide customers with a choice ofeither legacy service over a Hybrid Passive Optical Network (HPON)transmission path using an HFC transmission path between the customerand the head end, or DOCSIS 3.1 service over FTTP. The use of standardbinary PON technologies such as EPON and GPON are well known, but thebenefits from DOCSIS 3.1 over FTTP are a new phenomenon. Consideredtogether with the fact that traditional binary PON systems arecompletely transparent, this means that the transformational technologydisclosed herein, by supporting the legacy HFC infrastructure, alongwith the binary PON technologies that use FTTP, is a truly HPONarchitecture.

Traffic Engineering Fundamentals

Referring to FIG. 1, Tom Cloonan developed several graphs known as“Cloonan's Curves,” which incorporate an observed phenomenon calledNielsen's Law. Nielsen's Law roughly states that the highest offeredInternet speed will increase at an annual growth rate of 50%. With themigration to DOCSIS 3.1, the capacity of a HFC plant is roughly 10 Gbps.As can be seen by the circled region in FIG. 1, the expected 50% growthhits the 10 G ceiling around the year 2024, less than a decade away,assuming continued 50% growth rate over the entire interval along with acorresponding migration to all-IP Video. While the time frame predictedin FIG. 1 may initially be disconcerting to cable operators who mightthink that HFC will have run out of capacity by that date, in realitythis interval only represents the Top Billboard Tier which is typicallyless than 1% of all subscribers. Hence this time frame merely indicateswhen the migration from HFC to FTTP migration must begin, where thechoice of when a CATV provider decides to begin migration within thatinterval is discretionary.

To predict the effect that the other 99% of the subscribers may have onthe transition to FTTP, FIG. 2 presents a Network Quality of Experience(NQoE) formula first posited in a network capacity model developed byMike Emmendorfer. The NQoE formula goals include: (1) achieving MaxService Tier even during busy periods; (2) allocating an appropriateamount of network resources; (3) network resources sufficientlyconfigurable to accommodate any data network; (4) accommodatingestimates of Service Tier and Traffic Growth Rates; and (5) achievingMax Service Tier through Next Network Capacity Adjustment. While thedetailed formula is extremely complex, the simplified version below hasbeen found to work quite well in most situations:

C>=(Nsub*Tavg)+(K*Tmax_max)   (Equation 1)

where C is the required Bandwidth Capacity for the service group, Nsubis the total number of subscribers within the service group, Tavg is theaverage bandwidth consumed by a subscriber during busy-hour, Tmax_max isthe highest Tmax offered by the MSO, K is a QoE constant (larger valuesof K yield higher QoE levels) and K values for typical scenarios willfall in the range K=1.0-1.2.

The first component (NSUB*TAVG) in Equation 1 represents the averagestatic traffic load and is a function of the number of subscribers perService Group (SG) and the average bandwidth per sub at busy hour. Thesecond component (K*TMAX_MAX) of Equation 1 is the headroom required forgood Quality of Experience (QoE). Tmax is the Maximum Sustained TrafficRate parameter for DOCSIS Service Flows. Tmax max is the highest Tmaxacross all Service Flows. It should be large enough to support a burstfrom the highest offered service tier. Many operators may choose a QoEconstant, K, equal to 1.2 to give themselves an additional 20% cushion.

Given equation 1, several example traffic engineering scenarios that canhappen over the next five years may be considered. For a present day HFCscenario, assume a top service tier of 300 Mbps (i.e. Tmax_max) with 500subscribers per SG and Tavg=400 kbps. This scenario requires 200 Mbpsfor the static traffic load and 360 Mbps for QoE headroom for a minimumcapacity of 560 Mbps. An operator might deploy 16 DOCSIS 3.0 channels(96 MHz) to support this scenario, which is shown on the left of FIG. 3.

A couple years later, the middle scenario in FIG. 3 supports a maxservice tier of 1 G with 250 subs per SG and Tavg=1 Mbps, a scenariothat requires almost 1.5 Gbps. This may be achieved, for example, bybonding 24 DOCSIS 3.0 channels (144 MHz) with 96 MHz D3.1 OFDM channels.By the end of the decade, as illustrated by the right hand scenario ofFIG. 3, operators may try to max out D3.1 CPE capabilities and offer a 3Gbps service tier. By then, Tavg might be 2 Mbps. This scenario requiresat least 4.1 Gbps of capacity, which might be achieved by bonding 24DOCSIS 3.0 channels (144 MHz) with a pair of 192 MHz D3.1 OFDM channels

Considering these three scenarios, the DOCSIS spectrum has soared from96 MHz, to 240 MHz in only a few years, and eventually to 528 MHz by theend of the decade. To control this spectrum growth, an operator might betempted to consider splitting service groups. However, splitting aservice group only impacts the average static load. As can be seen inFIG. 3, this becomes a smaller and smaller component of the trafficengineering. During this stage, it becomes more important to increasethe HFC spectrum (e.g. from 750 MHz plant to 1 or 1.2 GHz plant), whichstill might not be sufficient.

In order to arrive at a better alternative, the present inventors morecarefully evaluated the service tiers other than the Top Billboard Tier.Table 1 shows a representative sampling of various service tierbreakdowns from several major North American MSOs. The Top BillboardTier for this sample in 2014 was 300 Mbps and less than 1% of the substook this service. Looking at the other service tiers, roughly 14% werein the “Performance” Tier @75 Mbps with the majority of subscribers inthe Basic Tier (65% @25 Mbps) and Economy Tier (20% @5 Mbps).

TABLE 1 2014 Service Tier Mix, Rates & Growth 2014 Service Tier Levels %of Subs Tmax (Mbps) Tmax CAGR Top Tier - Billboard rate  1% 300 50%Performance Tier 14% 75 32% Basic Tier 65% 25 26% Economy Tier 20% 5 15%However, forecasts indicate that, while the Top Billboard Tier isexpected to grow at the Nielsen's Law 50% CAGR (Compounded Annual GrowthRate) described earlier, the other service tiers had a significantlylower growth rate. The lower the performance of the service tier, thelower the CAGR.

While this difference in CAGR among service tiers initially might becounterintuitive, a more careful consideration indicates that itactually makes sense. If all service tiers grew at Nielsen's 50% CAGR,then every installed cable modem would be obsolete within 2-3 years ofintroduction, requiring a staggering investment level. Since operatorscontrol the CAGR for each service tier, they effectively control howlong the cable modem technology stays viable in the field. Note furtherthat the Economy Tier could still be using DOCSIS 2.0 modems from over adecade ago. In 2014, a 16-channel bonded cable modem was probably usedfor the Top Billboard Tier of 300 Mbps. In a few years, once 1 G serviceis available on HFC, the 300M service with its 16-bonded channel modembecomes the Performance Tier. A few years after that, it will berelegated to the Basic Tier.

FIGS. 4A and 4B together show the Tmax growth for each service tier overa 20 year window. As indicated earlier, the Top Billboard Tier may beexpected to arrive at the approximate 10 Gbps HFC limit by 2024. ThePerformance Tier Tmax does not achieve 10 Gbps until 2033. By this time,these subscribers will need to be migrated to FTTP. Meanwhile both theBasic and Economy Tiers are well under the 10 Gbps limit twenty yearsfrom now. In this example, 85% of the HFC subscribers do not need tomigrate from HFC for more than two decades, which even this assumes thatgrowth rates continue unabated. Thus, within 10 years, HFCinfrastructure will be able to deliver an Ultra -HD video stream toevery person on the network, so further growth will have to be driven bysome future application.

Note that the approximately 10 Gbps HFC limit assumes that DOCSIS 3.1has been deployed and legacy MPEG spectrum has been recovered. If anoperator chooses to stay with DOCSIS 3.0 technology, then their bestTmax would be around 1 Gbps using 32-channel bonded modems. From FIG. 4,the Performance Tier now needs to move to FTTP by 2024 and the BasicTier (i.e. 65% of subs) need to be on FTTP by about 2030. Thisdrastically alters the HFC to FTTP migration plans.

Table 2 shows where each service tier might be by the end of the decade.With a 50% CAGR, the Top Billboard Tier might be at 3 G service rate.The Performance Tier with its approximately 32% growth now reaches ˜500Mservice rate, while the Basic Tier has grown to 100M and the EconomyTier is around 10M.

TABLE 2 2020 Service Tier Mix, Rates & Growth ~2020 Service Tier Levels% of Subs Tmax (Mbps) Tmax CAGR

 

Performance Tier 14%  500 32% Basic Tier 65%  100 26% Economy Tier 20% 10 15%

Referring again to FIG. 3, the Top Billboard Tier still fits within theDOCSIS 3.1 cable modem capabilities (i.e. 2×192 MHz OFDM channels), butnow requires the operator to have 4.1 Gbps of DOCSIS capacity to offerthis service tier. With the “Selective Subscriber Shedding” method, theTop Billboard Tier would be migrated from HFC to FTTP. Note that thistier is typically less than 1% of the total subscribers, so a 250subscriber SG might only have two or three subscribers in this tier, onaverage, that need to migrate.

With the Top Tier removed, the traffic engineering can be re-calculatedfor the remaining HFC subs. The static load is essentially unchangedsince so few subscribers have been removed. However, the QoE portion ofthe formula has been drastically reduced since the top service rate(Tmax_max) is now 500 Mbps instead of 3 Gbps. This means that theoperator now only needs 1.7 Gbps of DOCSIS capacity instead of theprevious 4.1 Gbps before subscriber shedding. This corresponds to asavings of 250-300 MHz of spectrum using DOCSIS 3.1 OFDM channels. Bymigrating the Top Billboard Tier to FTTP, the operator has effectivelyextended the life of the HFC for the remaining subscribers.

Network Capacity Modeling of FTTP Migration

A network capacity model of this service tier example, which assumes 128subscribers per SG, is shown in FIG. 5. This particular model migratessubscribers to FTTP starting with the highest available service tier asDOCSIS capacity exceeds 10 Gbps. The dark portion of each bar is the QoEelement driven by Tmax_max. The light portion of each bar is the staticload.

As can be seen in FIG. 5, Tmax dominates in the early years. The 50%CAGR on the Top Billboard Tier is evident in the growth through 2023. In2024, the Top Billboard Tier (i.e. <1% of subs) is shed to FTTP andthere is a drastic reduction in required DOCSIS capacity. The growthrate is now the slightly lower Performance Tier. The Performance Tier isfine using HFC thru 2028 but needs to migrate to FTTP by 2029. By 2029,there is only 15% of subscribers that must be shed from the HFC to FTTP.At this point, with the highest tiers moved to FTTP and continued growthin Tavg, the static load has now become the dominant piece of thenetwork capacity formula. Beginning in 2031, Basic Tier subscribersstart to be migrated to FTTP in order to reduce the static load. Hence,the light component of the bars in FIG. 5 start to drop as there arefewer and fewer subscribers still using the HFC network. FIG. 5 alsoassumes a fixed SG size of 128 subs. Once the static traffic load startsto dominate, it now becomes desirable to split SG size which will reducethe static traffic load.

FIGS. 6A-6D provide four charts corresponding to SG sizes of 256, 128,64, and 32 subs. As can be seen from this figure, service group size haslittle impact over the next 8-10 years on determining when the TopBillboard Tier needs to migrate to FTTP (32 subscribers per SG is onlyone year after 256 subscribers per SG). However, SG size is a big factoron when the static load starts to dominate. For 256 subscribers per SG,the static load becomes the dominant portion by the year 2023. For 32subscribers per SG, the static load does not dominate until thefollowing decade.

The foregoing discussion illustrates several things. First, migrating toDOCSIS 3.1 is important. FIG. 5 shows the relative HFC limits with bothDOCSIS 3.1 and 3.0. Having the capabilities of DOCSIS 3.1 greatlyextends the life of the HFC network. Second, the near term goal of CATVproviders should to increase spectrum for DOCSIS 3.1. This might meanupgrading a 750 MHz plant to 1,002 MHz or even 1,218 MHz plant. To offer1 Gbps downstream services over the HFC, the operator should alsoconsider an 85 MHz upstream split at this time as well. Given existingasymmetric traffic loads, the 85 MHz upstream spectrum should match wellwith a 1 GHz downstream spectrum. As the provider looks further into thefuture, the static load will start to dominate and SG splits may thenbecome more practical. As operators migrate the highest tiers to FTTP,they should keep in mind that they will eventually need to do some SGsplits on HFC as well.

Economic Impacts of Selective Subscriber Shedding

While the prior discussion centered on traffic engineering benefits ofthe Selective Subscriber Shedding method, it is also worthwhile toconsider the economic impacts of this strategy. FIG. 7 shows exampleplant upgrade costs for a suburban case study with a serving area ofalmost 1000 homes passed (HP). Specifically, a full FTTP upgrade iscompared to various HFC upgrades to 1 GHz/85 MHz. The HFC options showprogressively deeper fiber. N-300 has no more than 300 HP on any leg andis typically N+1 or N+2 with a very limited number of outlying homes atN+4. N-150 and N-75 continue to increase nodes and reduce SG size. Theseoptions all use existing node & amplifier locations. Finally, the N+0upgrade is almost a complete rebuild of the HFC with nodes put in newsites as needed. The N+0 upgrade averages about 60 HP per node.

As is shown in FIG. 7, the plant upgrade costs skyrocket as fiber goesdeeper. The ˜$30K upgrade cost of N+0 is more than twice that of theN-300 upgrade. The ˜$60K cost of FTTP is double the cost of N+0 and isfive times more expensive than the N-300 upgrade. A key reason on whythe FTTP is much more expensive is that a significant portion of thefiber installation is associated with the last drop cable over the lastcouple hundred meters.

With the Selective Subscriber Shedding method, an operator only needs todo the N-300 HFC upgrade in the near term, at substantially less costthan either N+0 or FTTP. The N-300 upgrade provides essentially the samespectrum as N+0, so this satisfies the short term needs when Tmaxdominates. With the cost savings, a handful of Top Billboard Tiercustomers can be given FTTP connections. Over the next decade, thePerformance Tier can be gradually migrated to FTTP. When this happens,the fiber will also be pulled to enable a fiber deeper HFC migration toN-75 or even N+0 when needed over ten years from now. This approachallows operators to grow slowly as needed and spread plant investmentsover a lengthy time window, yet still be prepared for fiber deep SGsplits when needed a decade from now.

In summary, selectively shedding subscribers from HFC to FTTP startingwith the highest service tiers, combined with DOCSIS 3.1 and 1 GHz/85MHz upgrades to maximize HFC capacity, provides a sensible transitionfrom HFC to FTTP and relieves pressure to reclaim legacy spectrum. Thismethod not only saves money, it adds decades of life to the HFC plantfor 80% to 95% of the total subscribers by being able to support Gbpsservices to the masses. Furthermore, if entertainment and Ultra-HD isall that Basic & Economy Tiers require, then these subscribers canpotentially reside on HFC forever.

DOCSIS 3.1 Overview

DOCSIS 3.1 is a key element in the foregoing method to extend the lifeof HFC for decades. Some critical technologies underlying the DOCSIS 3.1standard include: OFDM, LDPC Forward Error Correction (FEC), MultipleModulation Profiles in the downstream, and Time and Frequency DivisionMultiplexed (TaFDM) CMTS Scheduler.

DOCSIS 3.1 provides the following important benefits: (1) it is DOCSIS3.0 backwards compatible and operates in existing HFC plants withoutchanges; (2) it has Ultra-wide, variable width channels, e.g. 24-192 MHzdownstream channels and 6.4-96 MHz upstream channels; (3) its highermodulations yield increased spectrum capacity, e.g. 4,096-QAM(16,384-QAM optional) in the downstream direction, 1,024-QAM (4,096-QAMoptional) in the upstream direction with Bps/Hz gains of 40%-75%(downstream) and 66% to 100% (upstream); (4) it provides new spectrumavailability, i.e. an optional future spectrum of 1,218 MHz downstream,204 MHz upstream for 10+ Gbps (downstream), 1.8 Gbps (upstream) as wellas a robust OFDM+LDPC Leverages Roll-off region in existing plants (˜1Gbps possible); (5) OFDM+LDPC and TaFDM maximizes existing upstream(e.g. ˜250 Mbps in 42 MHz); and (6) DOCSIS 3.1 MAC enables bondingacross 3.0 SC-QAM+3.1 OFDM.

The DOCSIS 3.1 specification also requires that the first generationDOCSIS 3.1 cable modems must support two 192 MHz OFDM channelsdownstream and two 96 MHz OFDMA channels in the upstream. That meansthese DOCSIS 3.1 modems, once deployed in the field, will be capable ofproviding capacities of 5 Gbps downstream and 1.8 Gbps upstream.

DOCSIS 3.1 Capacity and Migration Examples

FIGS. 8A and 8B show that DOCSIS 3.1 greatly increases the potentialcapacity of HFC. Today's DOCSIS 3.0 cable modems are limited to 32×8configurations. The 32-bonded downstream channels enable just over 1Gbps of capacity. The 8-bonded upstream channels provide about 200 Mbpsof upstream capacity. From an HFC plant perspective, total capacity fortoday's HFC is the combination of both the DOCSIS 3.0 channels and theMPEG Video QAM channels, which is represented in FIG. 8A with the3.0+QAM bars.

For a 750 MHz HFC plant, the downstream capacity goes from approximately4 Gbps for 3.0+QAM to approximately 7 Gbps for DOCSIS 3.1. For a 1 GHzHFC plant, this differential may go from approximately 5 Gbps for3.0+QAM to almost 9 Gbps for D3.1. Finally, DOCSIS 3.1 can provide over10 Gbps of downstream capacity over 1,218 MHz of spectrum.

Also of importance is the procedure by which migration from DOCSIS 3.0to DOCSIS 3.1 occurs. Initially, no HFC plant changes are needed. DOCSIS3.1 can be introduced into existing plants, providing capacity gainswith improved spectral densities, with the challenge being findingavailable spectrum for DPCSIS 3.1. In the downstream direction, DOCSIS3.1 provides am additional benefit in that it can operate in the roll-off region. For example on a 750 MHz plant, an OFDM channel could beplaced from 750 to 900 MHz. An analysis of an actual 870 MHz plantshowed that there may be as much as 1 Gbps of capacity in the roll -offregion, but this may vary substantially from one HFC plant to another.

At the point an operator does decide to upgrade, several options exist.The first option is to expand the existing HFC spectrum. For example, itmay be preferable to extend the downstream spectrum to at least 1,002MHz since such upgrades are straightforward and cost effective. Someoperators may consider going to 1,218 MHz but this will introduce someadditional challenges, especially considering power and tilt as well aspotential MoCA interference. When upgrading the HFC downstream spectrum,the operator may also consider increasing the upstream split to 85 MHz.This will help future-proof the HFC from an upstream capacityperspective.

The second upgrade option is to migrate select subscribers to HPON,which will give these HPON subscribers immediate access to expandedspectrum (e.g. 1,218 MHz downstream, 204 MHz upstream) while notrequiring any immediate changes to the existing HFC. Those of ordinaryskill in the art will appreciate that different operators have their owncircumstances that will dictate which upgrade option is selected, and/orwhich order they are applied. It may be that many operators will pursueboth options in parallel.

The final step in the migration to DOCSIS 3.1 should preferably be forthe operator to enable all IP video so that legacy MPEG spectrum can bereclaimed, and the entire HFC spectrum utilized by DOCSIS. The IP videodeployment should leverage the latest Multicast adaptive bit rate (ABR)protocols to make the most efficient use of capacity.

The migration sequence just described will allow operators to grow theirDOCSIS capacity on HFC from 1 to 2 to 5 to 10 Gbps over time.

Hybrid PON (HPON)

HPON comprises an innovative fiber splitter technology that completelyeliminates Optical Beat Interference (OBI) for RFoG wavelengths. HPONrequires minimal power, e.g. approximately 150 mW per drop connection.While not a purely passive architecture, HPON is not meaningfullydifferent from many PON installations that require PON extenders orRemote OLT at much higher power consumption. Thus, while minimal poweris needed for RFoG wavelengths, HPON is still essentially passive andcompatible with Ethernet and PON technologies such as 10 G Ethernet,EPON, 10 G EPON, GPON, NG-PON2. Hence, even if the RFoG wavelengths losetheir power, PON and Ethernet may continue to operate.

HPON is standards-compliant on both ends of the network. HPON iscompletely backwards compatible with today's RFoG ONU and RFoG HeadendOptics. An operator may use any vendor's RFoG-compliant ONU or Optics.Because HPON eliminates OBI, an operator is free to choose any vendor'sCMTS/CCAP with its traditional upstream scheduler. HPON enables fullDOCSIS 3.1 performance with an OBI-free environment. In contrast, someother RFoG solutions needed a specialized CMTS 3.0 scheduler, whichhandicaps performance and is not usable with DOCSIS 3.1.

HPON is “hybrid” in several senses. First, HPON supports legacy HFCservices Over FTTP. Second, HPON provides both DOCSIS 3.1 content alongwith traditional binary PON (e.g. EPON, GPON, NG-PON2). Third, HPONprovides DOCSIS content over both HFC & FTTP. Fourth, HPON provides bothDOCSIS 3.0 and DOCSIS 3.1 content. Fifth, HPON is both powered andpassive. Sixth, HPON provides both a symmetric and asymmetricapplications.

Fiber to the Home (FTTP) Transition

Until recently, EPON or GPON seemed to be the only reasonable long termFTTP choices. The DOCSIS over RFoG alternative was hampered by OpticalBeat Interference (OBI) as previously discussed. With HPON, DOCSIS 3.1over FTTP becomes a viable long term option. Those of ordinary skill inthe art will appreciate that this is not an either/or choice for theoperator, as HPON supports both EPON/GPON AND OBI-Free DOCSIS 3.1. Theoperator can support DOCSIS &/or EPON/GPON as needed, whichever is bestsuited to the service needs. For example, an operator might deploysymmetric 10 G EPON over HPON for Business Services while DOCSIS 3.1over HPON for Top Tier residential customers.

When considering DOCSIS or EPON over FTTP, operators should evaluateseveral factors. For example, EPON leverages Ethernet the ecosystem,supplies more than abundant bandwidth capacity up front, and offerssymmetric capabilities. From a MAC perspective, it is simple and relieson small service groups and polling for access.

DOCSIS fits seamlessly into HFC infrastructure, being spectrum friendly.It supplies bandwidth capacity as needed, i.e. “Just in Time”. This wasevident with DOCSIS 3.0 as the number of bonded channels grew over timewhile always being backwards compatible. HFC and hence DOCSIS has had anasymmetric focus on residential applications. The MAC is full-featuredto provide guaranteed services to very large service groups. Forexample, early DOCSIS adoption involves some service group sizes greaterthan 1000 modems.

HPON and the Role of DOCSIS

HPON support for DOCSIS over FTTP brings provides an operator with manypotential benefits. First, it leverages the existing DOCSIS/HFCInfrastructure, which allows both CCAP and DOCSIS CPE investments to bereused in an HPON architecture. DOCSIS 3.1 over HPON supports legacyMPEG Video services, meaning that operators can reuse legacy STBinvestment in the field.

HPON unleashes DOCSIS 3.1 capabilities to its full extent, providingPON-like Gbps data rates for both downstream and upstream directions.Initial DOCSIS 3.1 modems will have 5 Gbps downstream, 1.8 Gbps upstreamcapacities, which will enable true 1 G upstream services, unlike 1 GEPON, GPON or 10 G/1 G EPON which lack sufficient QoE upstream capacity.

By leveraging the DOCSIS MAC capabilities, DOCSIS 3.1/HPON supportsexisting service group sizes and distances, which are significantlylarger than traditional PON. DOCSIS is designed to handle 80 kmdistances with potentially 1000 modems, while traditional PONs arelimited to 20 km and 32-64 ONU. This conserves trunk fibers andwavelengths as well as CCAP ports. In the future, DOCSIS 3.1 OFDMtechnology in an OBI-free environment offers the potential of 40 Gbpsdownstream, 10 Gbps upstream on single wavelength.

Mixed HFC and HPON DOCSIS 3.1 Operation

In the disclosed Selective Subscriber Shedding method, there may only bea few Top Tier subscribers on the FTTP in a serving area. From theperspective of Head End infrastructure, this appears wasteful andexpensive if an entire CCAP or OLT port must be dedicated this smallnumber of customers. DOCSIS 3.1 over HPON can overcome this hurdle byreusing the same CCAP port that is being used by the HFC plant.

FIG. 9 shows an example of how the HFC and HPON spectrum can overlap andbe shared from a single CCAP port. The example of FIG. 9 assumes thatmost of the existing 750/42 MHz HFC spectrum is being used for DOCSIS3.0 and legacy QAM services, which in some embodiments might include24-32 3.0 channels. A 96 MHz 3.1 OFDM channel is placed on the HFC from738 to 834 MHz so it only replaces two QAM channels and leverages the750 MHz roll-off. This is enough DOCSIS capacity to offer 1 G downstreamservices and 100M upstream services (within 42 MHz).

Because HPON is full FTTP, it can support 1,218 MHz downstream. The CCAPport can put two additional 192 MHz OFDM channels from 834 to 1,218 MHz.This spectrum can then be sent down both the HFC and HPON. HFC modemswill only use the 96 MHz OFDM bonded with 3.0 channels and ignore thetop 2×192 OFDM. The HPON modems can bond across all OFDM and 3.0channels as needed, which could enable a 2.5 G or even 3 G downstreamservice.

Another significant advantage of HPON is the isolation betweendownstream and upstream spectrum, each with its own dedicatedwavelengths. This provides the operator with a cost effectiveoperational mechanism for migrating select customers to a DOCSIS 3.1 204MHz, true 1 G upstream services while keeping the vast majority ofsubscribers on existing HFC. Over time, this capability can also enableExtended Spectrum RFoG with significant bandwidth capacity enhancementsin both upstream and downstream. At the Head End, the 204 MHz HPONupstream can be combined with the 42 MHz HFC upstream and use the sameCCAP port. The 42 MHz spectrum is shared between HFC and HPON while42-204 MHz is available to HPON 3.1 modems. It should also be understoodthat HPON provides improved upstream Signal to Noise Ratio (SNR) andreduces upstream noise funneling from ingress in the home which shouldmake 4096-QAM modulation a reality in the upstream.

The overlapping spectrum has some additional benefits. Because thedownstream spectrum can stay within the 54-1,218 MHz band, it cancontinue to support legacy services such as STB in the lower spectrum.Thus, a benefit of HFC and HPON spectrum overlay on the same CCAP portis that a small number of subscribers can be cost-effectively moved toFTTP. This may be accomplished by simply licensing additional D3.1 OFDMchannels, and without additional hardware. This is in contrast to a PONmigration, where moving a small number of subscribers to FTTP mighttrigger the installation of an entire OLT where there may have been nonebefore.

Comparing DOCSIS 3.1 over HPON Capacities to Other PON Architectures

Operators have many different potential network options available tothem and their competitors, so it is important to understand how thesevarious technologies stack up against each other. A comparative chart ofdownstream capacities is shown in FIG. 10. Note that PHY Layer Rates arethose after encoding and forward error correction (FEC) if used. Copperbased infrastructure has made significant progress over the years andVDSL2 and G.fast are the current state of the art. Some estimates forthese copper solutions are also shown in FIG. 10. These copper solutions“only” provide hundreds of Mbps of downstream capacity to the user, notGbps as in the other solutions.

The traditional PON technologies include GPON, 10 G EPON and XG-PON.GPON provides almost 2.5 Gbps DS while 10 G EPON and XG-PON provides˜8.7 Gbps of DS capacity. “10 G” is a bit of a misnomer as it losesabout 13% of capacity to the FEC. NG-PON2 was not included in FIG. 10 asit is a multi-wavelength technology, whereas FIG. 10 shows what can bedelivered to a user with a single wavelength.

For DOCSIS on HFC, FIG. 10 shows the capacity for a 750 MHz plant with3.0+QAM (4 Gbps); 750 MHz with DOCSIS 3.1 (7 Gbps); and 1 GHz plant withDOCSIS 3.1 (8.9 Gbps). Note that a 1 GHz plant with DOCSIS 3.1 isroughly equivalent to a 10 G EPON downstream capacity. Finally, DOCSIS3.1 over HPON provides almost 12 Gbps of capacity in 1,218 MHz, which is33% more downstream capacity than 10 G EPON.

TABLE 3 Mapping D3.1 to PON Equivalents, Downstream Capacities NominalData Downstream Spectrum Capacity PON Equiv 30 ‘3.0’ + 96 MHz OFDM 2Gbps GPON, 2 × 1 G EPON 30 ‘3.0’ + 2 × 192 MHz 5 Gbps 2 × GPON, ½ 10 G30 ‘3.0’ + 4 × 192 MHz 8.7 Gbps 10 G EPON, XG-PON1, NG-PON2 12 24 × 192MHz ~20.40 Gpbs NG-PON2 (multiple λ)

Table 3 shows a mapping of downstream capacity for various DOCSISconfigurations into traditional PON systems. A DOCSIS system with 30 3.0channels bonded with 96 MHz 3.1 OFDM channel provides about 2 Gbps, isroughly equivalent to GPON, and is double 1 G EPON. A 2×192 MHz OFDMwith 3.0 channels now provides almost 5 Gbps, which is twice GPON butslightly more than half of 10 G EPON. As the number of OFDM channelsgrow over time, just as DOCSIS 3.0 channels grew, a 4×192 MHz OFDMbonded with 3.0 channels is equivalent to 10 G EPON downstream. Finally,Extended Spectrum DOCSIS 3.1 can achieve up to 40 Gbps DS over a singlewavelength. This downstream capacity would be equivalent to NG-PON2which would require 4 wavelengths for the same capacity.

TABLE 4 Mapping D3.1 to PON Equivalents, Upstream Capacities NominalData Upstream Spectrum Capacity PON Equiv 85 MHz OFDMA 750 Mbps 1 G/1 G10 G/1 G EPON, GPON HPON 204 MHz OFDMA 1.8 Gbps EPON w. 10 G/1 Gco-exist, XG-PON1, NG-PON2 (2.5 G) HPON 500 MHz OFDMA ~5 Gbps EPON w/ 10G/1 G co-exist HPON 1.2 GHz OFDMA ~10 Gbps 10 G/10 G EPON, NG-PON2

Table 4 shows upstream capacity mapping. An 85 MHz DOCSIS 3.1 HPONsystem upstream capacity is roughly equivalent to 1 G EPON, 10/1 G EPON&GPON with usable capacity in the 700-800 Mbps range. The 204 MHz DOCSIS3.1 system is roughly equivalent to XG-PON 2.5 G US. This specificationlater shows that this also matches 10/10 +10/1 EPON co-existence undercertain traffic conditions.

HPON Topology Options

The specification so far has discussed migration from HFC to FTTP. Asoperators start to consider delivery of multiple Gbps services to everyhome, a migration to FTTP would require a PON ONU to be in the premise,since copper drop cable technology has limited bandwidth and prevents aFTTC approach with PON. However, because coax is an effective drop cabletechnology that can support more than 10 Gbps to each home, it isappropriate to consider other fiber deep architectures besides FTTP.

First, DOCSIS 3.1/HPON could deploy a Fiber to the Curb or Tap (FTTC)architecture, which is depicted in FIG. 11. New deployment technologiesare now available that allow fiber strands to be economically insertedinto a coax conduit. FIG. 11 shows an FTTC architecture 10 where eachHPON ONU at a Tap location 12 drives coax drops to groups 14 of fourhomes. This approach saves the cost of pulling fiber drops to each homeand shares the cost of ONU across multiple homes.

Another DOCSIS 3.1/HPON topology would be Fiber to the Multi-DwellingUnit (MDU). The ONU could be located in the basement or supply room andleverage existing coax distribution throughout the building. Alternatelyfor a larger MDU, the fiber could be pulled to every floor where asingle ONU serves the entire floor via coax.

HPON Topology Migration First Example

An HFC to HPON migration example is provided in FIGS. 12-18 to betterunderstand the various Topology options. FIG. 12 shows the baseline ofan existing HFC Plant 20 where a fiber trunk 22 feeds signals to andfrom a fiber node 24. The fiber node 24, in turn is connected to atree-and-branch network of amplifiers 26 at customer premises throughcoaxial cables 23.

The first step of the migration is shown in FIG. 13 where selectcustomers 28 are migrated to FTTP using an HPON unit 32. This migrationcan be achieved with either DOCSIS or PON. In FIG. 13, as an example, abusiness is connected with 10 G EPON and a Top Billboard Tier user getsa D3.1/HPON FTTP connection to their home. To perform this migration forselective customers, the coaxial cable lengths to the select customers28 are replaced with a fiber connection 34 and Fiber-Deep nodes 30,which may be upgraded to 1 GHz/85 MHz. As operators pull fiber to theTop customers, they will most likely pull additional dark fibers aswell. This will enable a future fiber deep migration along this path.The remaining customers in the service group may remain connected to thenode 24 via the existing amplifiers 26.

HPON provides an additional benefit for Fiber Deep deployments.Traditionally, every Fiber Deep node would need its own set of Head Eendoptics and require a separate wavelength on the fiber trunk. Byleveraging RFoG optics, the HPON system can act as an aggregator forFiber Deep nodes, and they can reuse the same optics being used for theD3.1/HPON FTTP home. This makes Fiber Deep more economical.

FIG. 14 illustrates the second step in the migration, where more Topcustomers 28 are migrated to FTTP using HPON. This example showsadditional 10 G EPON users, along with DOCSIS 3.1 users. FIG. 14 alsodepicts D3.1/HPON being delivered to an MDU 27 as well as Fiber to theCurb 29 being shared by several homes.

As the Performance Tier is migrated to FTTP (e.g. 5% to 15% ofsubcribers), then most of HFC plant will be covered by Fiber Deep. InFIG. 14, there are only a couple stray amplifiers 26 left without fiberbefore the entire HFC can be converted to Fiber Deep. Note that all ofthe Fiber Deep nodes are still sharing the same single set of RFoGoptics.

The third step of the migration shown in FIG. 15. At this point, theFiber Deep HFC has been completely built out and the Top Tier customersmoved to FTTP. Eventually, the static traffic load will increase as inFIG. 6 and the operator will need to split service groups, as needed. Atthis point, all service group segmentation is localized to the HPONsplitter and multiple wavelengths came be sent down for the differentSG. In FIG. 15, service group segmentation becomes simple, and isanalogous to node segmentations done today.

With this strategy, the operator only needs to deploy as many CCAP portsand Head End optics as is warranted based on subscriber demand, and thengrow these over time as demand requires. This is exactly the DOCSISphilosophy.

HPON Topology Migration Further Examples—Remote Devices

The HPON architecture has a primarily passive Outside Plant with itsreduced operational expenses while maintaining a traditional centralizedHead End architecture. An alternative approach is the Distributed AccessArchitectures (DAA) where intelligent devices such as Remote PHY, RemoteMAC+PHY and/or Remote OLT are pushed out to the nodes in the plant. ButHPON and Remote Devices are not mutually exclusive. A key motivation forRemote PHY Devices (RPD) and Remote MACPHY Devices (RMD) is theelimination of long analog AM optic fiber links enabling higher D3.1capacities. The deployment of RPD/RMD is often considered with FiberDeep upgrades as well.

Conventional wisdom today places the RPD/RMD at the Fiber Deep Nodelocation. Starting from the baseline example in FIG. 12, the distributedarchitecture might appear as in FIG. 16. There are now twelve RemoteDevices 38 (RPD/RMD 0 to RPD/RMD 11) in the serving area, and each mightonly be serving ˜60 HP or only ˜30 subscribers. Based on previoustraffic engineering results, Remote Devices will have excess capacityfor another decade or two.

HPON enables an alternate distributed architecture with shared RemoteDevices. This is shown in FIG. 17. This figure shows that HPON migrationsteps 1 and 2 have been completed. Top customers have received FTTP andthe Fiber Deep nodes are aggregated using HPON 42.

The difference between FIG. 14 and FIG. 17 is that the previousconnection from the HPON splitter to the Head End optics and CCAP overthe Fiber Trunk has now been replaced with a connection to a singleRPD/RMD remote device 38 a that is logically placed near the HPONsplitter. The Remote Device must now contain short distance AM opticmodules that support distances less than a kilometer. This particularexample shows a single 2×2 RPD/RMD that can support 2 service groups 40a and 40 b.

The key benefit to the architecture shown in FIG. 17 is that it onlyrequires a single Remote Device compared to a dozen devices required ina typical distributed system shown in FIG. 13. HPON ONU is significantlyless complex than RPD/RMD devices which save the operators significantcosts and power at every Fiber Deep node location.

Eventually, the time will come where the service size needs to be split.In a shared Remote Device scenario, the additional resources can beadded at the same location as the original Remote Device. This devicemight be upgraded from a 1×1 or 2×2 RPD/RMD to a 4×4 or 6×6 or 8×8device. Since this upgrade will occur many years in the future, thiswill be done with much newer technology thanks to Moore's Law and givethe operator substantial cost and power savings per SG. This example isshown in FIG. 18.

The approach shown in FIGS. 17-18 does not preclude adding other RemoteDevices in other locations. For example, maybe there is a neighborhoodhotspot or an MDU 44 that deserves its own Remote Device. FIG. 18 showsan additional RPD/RMD 38 b being added for the MDU 44.

DOCSIS 3.1 and RF Performance over HPON

To verify the potential of DOCSIS 3.1 over HPON, several measurementswere taken that analyzed the RF upstream performance. FIG. 19 shows aset of MER curves for various RF spectrum loads on a first generationHPON splitter system over 20 km. For this system, a reverse transmitterwas modified to go up to 1.2 GHz in the upstream, which then fed theHPON splitter. When viewed in the context of what SNR is required byDOCSIS 3.1, one can easily see that a 204 MHz spectrum can easilysupport 4K QAM, with very good SNR for higher frequency spectral load.

When the above SNR graph is converted to the capacity available, asindicated in FIG. 20, the capacity available is a monotonicallyincreasing function of bandwidth, and at the 1.2 GHz upper limit,provides for almost 10 Gbps of upstream data throughput. At a moremodest RF bandwidth of 204 MHz, the HPON system provides 2 Gbps ofcapacity. This is compared to SC-QAM technology @64-QAM which is whatDOCSIS 3.0 uses today. By way of comparison, current 42 MHz DOCSIS 3.0 4channel bonded system provide only 100 Mbps of throughput.

FIG. 21 shows HPON downstream performance where a DML transmitter wasmodified to 2.5 GHz of spectral load. Over 20 km of fiber, thedownstream receiver produced a corrected MER that enabled 2K QAM formuch of the spectrum and 1K and 0.5K QAM for the remaining portion. Whenthe downstream capacity is now computed for various spectral loading,one can see from FIG. 22 that the HPON capacity can approach 20 Gbps fora 2.5 GHz spectral load. This figure also shows how OFDM capacitycompares to SC-QAM channels@256-QAM.

By enabling OBI-free DOCSIS 3.1 over HPON, a variety of options becomesavailable to operators. HPON unleashes DOCSIS 3.1 capabilities to offerPON-like Gbps services in both upstream and downstream. It leverages theDOCSIS infrastructure making it very cost effective for incrementalinvestments for a gradual HFC to FTTP migration. It also makes availablenew potential HPON topologies such as FTTC, MDU and N+0.

Since DOCSIS supports large service groups, it enables fiber andwavelength conservation in the plant and allows the CCAP port costs tobe amortized over a larger number of users. Having significantly fewerCCAP ports also helps with head end space and power considerations.

10 G & 1 G EPON, GPON on HPON: Residential Scaling Considerations

The foregoing discussion of the benefits of DOCSIS 3.1 over HPON revealsthe ability of traditional PON, when used in accordance with thedisclosed systems and methods, to handle larger SG, and in particularlarger residential SG.

To better understand PON upstream capacity, FIG. 23 shows an exemplaryEPON burst schematic 50, which shows the various overheads associatedwith each upstream transmission burst. Of particular note is the laserturn on and turn off times at the ONU, and the Automatic Gain Control(AGC) and Clock Data Recovery (CDR) times required by the OLT receiver.It turns out that for a 1 G EPON upstream, the total burst overhead isin the range of 1.5 to 2.1 microseconds, which corresponds to anoverhead of 188 to 264 bytes for every transmit burst.

As EPON evolved to its 10 G upstream, the transmitter burst overheadswas reduced, but not by a factor of ten. For a 10 G upstream, thetransmitter overhead may vary from0.6 to 1.6 microseconds, whichcorresponds to an overhead of 764 to 2000 bytes for every transmitburst. A survey of industry literature by Glen Kramer, et al, shows howthe EPON upstream is impacted by the number of ONU and LLID and GrantCycle Time. The Grant Cycle Time is the frequency of the OLT polling ofeach LLID in the ONU. This results in a 64 byte Report message beingsent in the upstream direction Tables 5 and 6 show some results for a 10G upstream.

TABLE 5 10 G EPON Upstream Efficiencies ONU × LLID 1 ms 2 ms 4 ms  3285.00% 86.05% 86.57%  64 82.91% 85.00% 86.05% 128 78.72% 82.91% 85.00%

TABLE 6 10 G EPON Upstream Capacities ONU × LLID 1 ms 2 ms 4 ms  32 8.47Gbps 8.59 Gbps 8.65 Gbps 128 7.78 Gbps 8.24 Gbps 8.48 Gbps

As can be seen for the parameters tested, efficiencies varied from ˜79%to 87%. It is noted that the FEC accounts for 13% overhead. This meansthat the TX burst overhead varies from 0.5% to 9% based on these inputparameters. This shows that EPON TX Burst overhead is very sensitive toONU, LLID and Grant Cycle Time.

Extending 10 G EPON Capacity Analysis

Considering the previous discussion of the disclosed SelectiveSubscriber Shedding method, an operator might only need a service groupsize of 250 subscribers for the next five to seven years. A largeservice group size would minimize the number of OLT ports and fibertrunks required. However, each ONU might also have four to eight LLIDassociated with it. This implies that the product of ONU×LLID might goup to 1024.

The present inventors recreated and simulated the Kramer analysis over awider range of parameters. The results are shown in FIG. 24. It shows aset of curves with different Cycle Times that fall off rapidly withincreasing ONU×LLID. For example, 512 LLID (e.g. 64 ONU with 8 LLIDeach) with a 1 msec Cycle Time (needed for voice, gaming & MEFapplications) has capacity of only ˜5 Gbps.

Given this sensitivity to transmitter burst overhead, a closerevaluation of the parameters was made to determine a reasonable set forfurther testing. While an ONU might support 8-16 LLID, many will not beactive and not require any polling. Based on DOCSIS experience, thepresent inventors determined that 4-5 active LLID per ONU would bereasonable.

The DBA scheduler in EPON also has the capability to poll each LLID atdifferent intervals. Our analysis assumes that one LLID is needed forlow latency applications with a 1 msec Cycle time, while another 4 LLIDmight have an average cycle time 4 msec. Since EPON allows multipleReports per transmitter burst, our model assumes that there would be onaverage one transmitter burst per millisecond with an average of twoReports per transmitter burst.

1 G EPON, 10 G EPON and GPON Efficiency

10 G EPON has a 10 Gbps downstream PHY rate, but supports two differentupstream PHY rates: 1 Gbps and 10 Gbps. These are often referred to as10/1 and 10/10 EPON. The control overhead efficiency is calculated andshown in FIG. 25. The control overhead efficiency is basically thepercentage of time available to transmit after the polling overhead. Itexcludes the FEC overhead for the 10 G upstream. The efficiency iscalculated for both 1 G and 10 G upstream, and for the min and maxtransmitter burst overhead. As can be seen from FIG. 25, 1 G upstreamloses 28% to 36% capacity for 128 ONU while 10 G upstream loss is in the9-23% range for 128 ONU.

The chart also shows the efficiency for the GPON upstream. GPON is asynchronous system with only a 2 byte status report that is sampledevery 125 microseconds. GPON efficiency is close to the 10 G best case.

EPON: 1 G and 10 G Coexistence, Control Overhead Impact on Efficiency

10 G EPON supports the feature of simultaneously allowing 10/10 and 10/1ONU to share the same OLT port. This is very desirable from anoperator's perspective as they can deploy lower cost 10/1 ONU inasymmetric applications like residential while more expensive 10/10 ONUare deployed to symmetric applications like business services. Otheroperators may decide to deploy cheaper 10/1 ONU today and then in thefuture deploy 10/10 ONU once they are more cost effective.

However, coexistence can have significant impact on upstream efficiencyand capacity. Since the 10/10 and 10/1 share the same OLT port, only onecan be transmitting at a given time. This is analogous to the 802.11scenario where 11b and 11g WiFi devices coexisted in the same spectrum.The slower 11b devices took so much transmit time it left littlecapacity for 11g devices. 10 G EPON concerns are potentially worse, asthe difference in speeds is now a factor of ten.

There are two key factors when analyzing the 10/10 and 10/1 coexistencescaling. First, the control overhead is a function of the ONU mix. Theefficiency becomes a blend dependent on the ratio of 10/10 ONU and 10/1ONU. The second key factor is the traffic mix between 10/10 and 10/1ONU. It is assumed that 10/10 ONU will provide a higher upstream trafficload than 10/1 ONU.

With these factors in mind, three different scenarios were considered:

Scenario 1: 50% of ONU are 10/10, 50% 10/1; Traffic Mix is 90% 10/10,10% 10/1

Scenario 2: 25% of ONU are 10/10, 75% 10/1; Traffic Mix is 75% 10/10,25% 10/1

Scenario 3: 10% of ONU are 10/10, 90% 10/1; Traffic Mix is 50/50

FIG. 26 shows the results. For Scenario 1, network capacity is cut inhalf compared to 10 G-only upstream, even though 90% of the traffic iscoming from a 10/10 ONU. For Scenario 2, network capacity is only onethird compared to 10 G-only upstream. Finally in Scenario 3 where 50% ofthe traffic is coming from a 10/10 ONU, network capacity is less than 2Gbps, marginally better than 1 G-only upstream.

10/10 & 10/1 Coexistence Compared to GPON and D3.1/HPON

With such significant degradation in capacity caused by 10/10 & 10/1coexistence, it is useful to see how these scenarios fared when comparedto GPON and to D3.1 over HPON. FIG. 27 adds GPON to the ONU Mix shown inFIG. 26 and compares the results with Scenarios 2 and 3 of FIG. 26, aswell as 1 G EPON US. As can be seen, GPON handles larger ONU countbetter than EPON. GPON capacity is competitive with these mixed 10/10 &10/1 scenarios for large ONU counts.

FIG. 28, adds DOCSIS 3.1 upstream capacity to the ONU mix for both 85MHz HFC and 204 MHz HPON networks. As can be seen from this figure, theDOCSIS 3.1 Network Capacity is relatively independent of ONU count.DOCSIS 3.1/HPON outperforms 10 G EPON Scenario 3 with 50% 10 G upstreamTraffic. D3.1/HPON is comparable to Scenario 2 of FIG. 26 (75%10 G USTraffic) for many ONUs. DOCSIS 3.1 on 85 MHz HFC is comparable to 1 Gupstream, especially for larger ONU counts.

Residential Applications Present Traffic Engineering Challenges

As previously seen, 10 G EPON has significant TX burst overheads, up to764 to 2,000 bytes. This means that the average transmitter burst needsto be sufficiently large to minimize the effect of this overhead.However, this may be problematic in a residential environment.

With respect to residential traffic usage, packet size distribution isroughly 30% small packets (e.g. 64 B), and 70% large packets (e.g. 1,500B). There are a relatively small percentage of heavy users that accountfor majority of upstream traffic. Recent Sandvine data shows BitTorrentfile sharing as the leading upstream application; with the remaining topapplications related to real-time entertainment (e.g. Netflix, YouTube).Sandvine data also shows that traffic asymmetry actually increasesduring peak busy hours.

From these observations, several conclusions can be extrapolated. First.File sharing applications will be bursts of large packets from a limitednumber of users, with a good probability of bursts of large packetstogether. Second, real-time entertainment drives many small packets(e.g. IP Acks) from many users with little chance of bursts of more thana couple small packets together. Since video is driving the bandwidthgrowth engine, this traffic mix is not likely to change anytime soon.

FIG. 29 shows the average transmitter burst size required for 100%utilization for upstream traffic load spread evenly across all ONUs.Looking at 64 ONU with 100% 10 G upstream, each ONU needs a 16 KBaverage transmitter burst size each millisecond from every ONU tomaintain 100% utilization. FIG. 30 shows the average transmitter burstsize for heavy users required for 100% utilization with a packetdistribution based on the extrapolations above. It turns out that with64 total ONU (100%10 G), of which 8 are heavy users, the heavy usersneed to have a 117 KB average burst size every millisecond to maintain100% utilization of the 10 G upstream.

These results show that 10 G EPON will need extremely large transmitterburst sizes in order to maintain its utilization, which becomessignificantly worse when a packet distribution from a residential usecase is factored in.

The Role of FTTP and Hybrid PON—Other Considerations

Much of the preceding discussion in the present disclosure has focusedon the capacity of an HPON system. It is also important to consider theeconomics of HPON. The present inventors completer an analysis thatincluded total system costs including fiber deployment, ONU, andCCAP/OLT along with associated optics. The 1 G EPON case was used as abaseline for a relative system cost comparison. The results are shown inFIG. 31.

The top two curves compare 10/1 EPON costs to a D3.1/HPON FTTP costs.Both are assumed to have 1 user per ONU. Both are about 2½ times thebaseline cost of 1 G EPON. The DOCSIS 3.1/HPON costs are slightly lessthan 10/1 EPON as it can reuse existing HFC CCAP ports.

The bottom two curves show 1 G EPON compared to D3.1/HPON FTTC costswith 4 user per ONU, and are very close to each other cost-wise. TheHPON FTTC approach generates significant savings by eliminating the needfor a fiber drop to the end user and by sharing the cost of the ONUacross four users. With HPON FTTC, an operator ends up with 10/1 EPONcapacity at a cost of 1 G EPON. This also highlights the HPON FTTCsavings when compared to HPON or 10/1 EPON FTTP costs.

Fiber Trunks, Wavelengths and OLT/CCAP Ports

For the HFC to FTTP migration, most operators will plan to reuse theirexisting fiber resources as much as possible and focus investment onpushing the fiber deeper towards the home. Many operators have limitedfiber between their head ends and hubs to their serving areas, so bothfiber count and wavelengths are critical resources. There are also headend space and power considerations based on the number of OLT/CCAP portsrequired.

FIG. 32 plots the number of fiber trunks and/or wavelengths required doespecified numbers of users, which also corresponds to the number ofOLT/CCAP ports that are needed. A traditional PON system at maximum 20km distances would typically have 32 users per SG/OLT port. This servicegroup size is often limited by the fiber loss budget. For every 32 usersin a service group, another fiber trunk is needed as well as another OLTport. For 512 users in a serving area, the traditional PON system wouldneed 16 fiber trunks and 16 OLT ports.

An alternative PON approach is to use a PON extender or Remote OLTtechnology. This will increase both the distance from the Head End aswell as service group size. But the increasing service group size needsto be balanced against the capacity efficiency concerns discussed in theprevious sections. FIG. 32 assumes the extended PON can support 64 usersper service group. This means a 512 user serving area would have 8service groups, need 8 wavelengths, and have 8 OLT ports.

DOCSIS 3.1 over HPON leverages the DOCSIS infrastructure and can supportlarge service groups. It might only need 1 or 2 service groups for aserving area of 512 users. That means only 1 or 2 wavelengths and 1 or 2CCAP ports are required. This saves the operator significant head endspace and power compared to PON approaches. At a later time whenadditional capacity is needed, then the service groups can be split andadditional CCAP ports and wavelengths added as needed.

HPON and Energy Considerations

Energy consumption is becoming increasingly more important. In reviewingthe different architectures, power consumption is shifted between thehead end and the outside plant (OSP). For traditional PON, 100% of theoperator's power is in the head end, but in a distributed R-CCAParchitecture, almost all of the power consumption is in the outsideplant.

To be able to compare these different architectures, it is important toconsider the total energy consumption. This must include both outsideplant and head end power impacts. FIG. 33 takes a look the relativepower consumption of various HFC, PON, and HPON alternatives. The powerconsumption is normalized on an annual cost per Homes Passed. The upperportion 52 of the bars represents the head end power while the lowerremaining portion of the bars represent power consumption in the outsideplant.

HFC systems 54 are on the left in the figure. A typical N+5 system isthe most power hungry of all the architectures. Most of the power isbeing consumed by amplifiers, actives and nodes in the outside plant.Next to that is the Fiber Deep N+0 HFC system 56. This reduces the N+5power consumption by more than 25%, but is still high compared to theother alternatives.

The PON systems 58 are next on the chart. The traditional PON has 100%of its power consumption in the head end. It is about half of the H+5HFC and 30% better than N+0. It is still relatively high because it islimited to 32 users per OLT port, requiring a large number of total OLTports. A PON system 60 with an extender continues to make improvements.By doubling the SG size to 64, the OLT port count and head end power iscut in half. This is offset slightly with some additional OSP power forthe PON extender. Finally, an estimate of a Remote OLT solution 62appears to provide the best total power consumption of the PON systems,but just marginally better than an extended PON.

Finally, the D3.1/HPON system occupies the two bars 64 and 66 on theright. One is an HPON FTTP topology and the other is FTTC with 8 homespassed per ONU. The head end power is the same as HFC, and leverages thefact that each CCAP port supports 256 users. For HPON FTTP, the OSPpower consumption is close to the same as head end power. Note that HPONFTTP power is less than 25% of the N+5 total power consumption androughly half the total power consumption of a traditional PON system andbetter than Extended PON or Remote OLT systems.

The HPON FFTC solution is the most power efficient end-to-end (E2E)system. By sharing a single ONU across 8 homes, the OSP powerconsumption becomes negligible. The HPON FTTC system is the most costeffective from both a CAPEX and OPEX perspective. Note that thisanalysis does not include the power for the ONU since that will often bepowered at the premise.

For existing plants, it has become clear that this is not a choicebetween HFC or FTTP. The transition will inevitably occur over manydecades, hence managing this on-going transformation effectively isdesirable. The disclosed Selective Subscriber Shedding methodintelligently moves Top Tier subscribers to FTTP in a manner that canadd decades to the life of HFC with 80% to 95% of all subscribers notneeding migration during that time. It is also economically prudent,showing where and when is best to invest in outside plant. A key pieceof this strategy is the use of DOCSIS 3.1 on HFC. This can increaseDOCSIS capacity by tenfold over 3.0 data rates. This is a criticalelement to make sure HFC remains useful through the FTTP transitionperiod.

For the FTTP transition, it has long been thought that traditional PONwas the only option. The present inventors realized that HPON, whichcompletely eliminates Optical Beat Interference (OBI), cansimultaneously support traditional PON such as 10 G EPON or GPON as wellas OBI-Free DOCSIS 3.1 over HPON. This splitter based technologysupports standard based components on either end of the network and iscompletely transparent. While EPON and GPON technologies are well known,the present disclosure describes this new DOCSIS 3.1 over HPON that willbe of great benefit to operators.

HPON unleashes the capabilities of DOCSIS 3.1. Operating in a FTTPenvironment allows full use of the spectrum in both the upstream anddownstream. Separate wavelengths allow spectrum overlap which enablesthe initial DOCSIS 3.1 modems to support 5 Gbps DS and 1.8 Gbps US, withhigher data rates expected in the future. The downstream capacity ofDOCSIS 3.1 over HPON is actually 33% more than 10 G EPON. The 204 MHzupstream capacity is twice that of 1 G EPON, 10/1 EPON and GPON. Itenables operators to offer a true 1 G upstream service which these otherPON technologies do not.

By leveraging coax as a high performance drop cable, HPON also enablesother fiber deep architectures besides FTTP. HPON supports Fiber to theCurb (or Tap), Fiber to the MDU (basement or floor) and even economicalFiber Deep nodes (N+0). An HPON architecture can also be used jointlywith distributed architectures to provide the best of both worlds: ashared Remote Device to amortize cost and lowest cost and power FiberDeep nodes.

Many of the advantages of using DOCSIS over HPON come from leveraginglarge service groups. Over the years, DOCSIS has been shown to scalenicely to many hundreds of modems and thousands of Service Flows. EPONefficiencies are very sensitive to the number of ONU, LLID and the GrantCycle time. Given a reasonable number of LLID per ONU and Cycle times tosupport low latency applications, it will be difficult to push an EPONsystem beyond 128 ONU.

10 G EPON supports a coexistence mode that can support 10/10 and 10/1ONU. While desirable from an operational point of view, there aresignificant potential negative performance impacts. A scenario with 50%10/10 ONU and 90% 10/10 ONU traffic will lose half its capacity to theslower 10/1 upstream. Another scenario with 10% 10/10 ONU and 50% ONUtraffic gets less than 2 Gbps capacity, which is less than a 204 MHzD3.1/HPON system.

Economics and energy consumption are two key factors to be considered indetermining the best solution path forward. In both cases, the HPON FTTCsolution leads the way in both cost and power. An optimum solution formany operators is one that can simultaneously support a mix of both RFoGand PON over a shared Optical Distribution Network (ODN). This gives theoperator total freedom to migrate subscribers between D3.1/RFoG and PONat their discretion as needs arise with minimal operational costs. Theoperator may always select the most appropriate technology.

HPON also enables other Fiber Deep topologies such as Fiber to the Curb(or Tap), Fiber to the MDU and even Fiber to the Deep Node (N+0). Byleveraging the extremely high bandwidth of existing coax as the finaldrop cable, eliminating the costs of pulling fiber over the drop cable,and sharing ONU costs across multiple homes; these other Fiber Deeptopologies with DOCSIS 3.1 (D3.1) over HPON provides operators with costeffective alternatives to pure FTTP. Finally, with DOCSIS 3.1capabilities unleashed over HPON, the present disclosure informs thoseskilled in the art how to decide when it is best to use DOCSIS 3.1 overHPON or traditional PON technologies, or a combination of both overHPON.

The HPON approach also supports overlapping upstream and downstreamspectrum that solves the issues around supporting legacy downstreamservices in the 54-258 MHz range. However, this FTTP can be much morecostly than an HFC based approach. The challenge becomes how to have theHPON FTTP flexibility of upgrading individual homes or very small groupsof homes to the extended US splits while being cost effective withtraditional HFC systems with coax drops to the home.

Disclosed herein are techniques for leveraging the HPON architecturewith support for overlapping US and DS spectrum. However, rather than aFTTP topology that pulls fiber all the way to the home, HPON is used ina fiber deep topology while still using coax as the final drop to thesubscriber. This includes topologies such as Fiber to the Curb (FTTC),or Tap, or MDU (e.g. basement or floor) or Fiber deep node. The HPONarchitecture now becomes cost effective as fiber no longer needs to beinstalled all the way to the subscriber home (i.e. reuse existing coaxdrops) and the HPON ONU costs are shared across multiple homes (e.g. 4-8coax drops per ONU for Fiber to the Tap).

Multiple Upstream Split Support

It is noted that prior art upgrades to HFC systems requires that allhomes in the same service group share the same upstream split. So evenif only a single subscriber needs the enhanced 204 MHz upstream, eachand every home also needs to be upgraded at the same time.Alternatively, U.S. Pat. No. 8,537,861, titled “SYSTEM FOR STACKINGSIGNALS IN AN EXPANDED FREQUENCY RANGE”, describes a system for RF blockconversion of the upstream spectrum at a particular location that passesthis upstream frequency back up the coax plant above the existingdownstream spectrum (e.g. move an 85 MHz US up to 1200-1285 MHz aboveexisting 750/870/1,002 MHz downstream). However, this approach requiresthat every active component in the coax path be upgraded with atriplexer; and it also now limits downstream spectrum expansion thatDOCSIS 3.1 may try to use in the future.

The present disclosure, however, takes HPON FTTC a step further bysupporting a novel configuration where each coax leg from an HPON ONUmay have its upstream split be individually selectable. That means thata single home on one coax leg of a service group can be upgraded to 204MHz split over coax while the rest of the service group remains at theexisting upstream split on the other coax legs.

FIGS. 34A and 34B, for example, illustrate a system capable ofconfiguring service of individual subscribers to respectively differentsplits between upstream and downstream signals. Referring specificallyto FIG. 34A, a single output Optical Network Unit (ONU) 100 may have anoptical transmission line 102, which carries upstream signals to a headend from a subscriber and downstream signals from the head end to asubscriber, connected to a Bidirectional Optical Subassembly (BOSA) 104that includes all elements (e.g. laser diodes, photodiodes,transceivers, etc.) necessary to both transmit and receive opticalsignals to and from the optical cable 102. In the downstream direction,the BOSA is connected to a downstream matching amplifier 106 thatamplifies the downstream signal from the optical cable, and in theupstream direction the BOSA 104 is connected to a laser driver 108 thatreceives broadband upstream signals from a subscriber and uses thosesignals to modulate the laser in the BOSA 104.

As noted previously, in prior art architectures, all subscribers in theHFC service group served by an ONU need to send and receive signalsutilizing the same split between upstream and downstream signal toprevent those signals from interfering with each other. Thus, shouldsuch a prior art system be upgraded to provide service to one subscriberin the service group so as to operate on a different, higher split toprovide more bandwidth for the upstream and/or downstream signals, allsubscribers in the service group would have to be upgraded to the newsplit, which would entail upgrading Customer Premises Equipment(CPE)—such as modems, cable boxes, etc—for every subscriber in theservice group.

The ONUs shown in FIGS. 34A and 34B, however, are capable of customizingthe connection to each subscriber 110 to its own split, independently ofwhatever splits other subscribers in service group are using.Specifically, the ONU 100 includes a split setting element 112 that iscustomized to the specific split used by the subscriber 110 connected tothe port 114 of the ONU 110. In FIGS. 34A and 34B the split settingelement 112 is shown as a diplexer, but the split setting element couldalso be any one of several devices, including a diplexer, a triplexer,and a directional coupler as explained below.

Generally speaking, two methods are available to separate upstream anddownstream traffic, at a given split, on a cable to going to asubscriber 110 (or between amplifiers in a network as needed). First,referring to FIG. 35A, a directional coupler 120 (or a circulator) is a3-port (or 4-port device) that has the property that signals are coupledbetween ports with a dependence on the direction of signal travel. Thedirectional coupler 120 is schematically shown in this figure as a4-port device. The subscriber coaxial cable 122 may be connected to port2. For forward (downstream) signals into port 1 (direction of arrow)signals are coupled from port 1 to both to port 2 and port 4, but not toport 3. For reverse (upstream) signals into port 2 (direction of arrow)signals are coupled from port 2 to both port 1 and port 3, but not toport 4.

The output from amplifier A goes from port 1 to port 2 and is put out indownstream direction on the cable 122 connected to the subscriber 110.The upstream signals from the subscriber are going into port 2 and areput out at ports 1 and 3, where port 3 can amplify the signal as anupstream signal using amplifier B. Port 4 may not be implemented. Thecoupler operates independent of frequency. Therefore downstream andupstream frequency spectra can overlap and the signals can still beseparated. Part of the upstream signal is also put out at port 1 and isnot used by amplifier A; this causes a loss of signal energy. This canbe solved by using a circulator, another directional RF device withslightly different properties, that couples all power into port 2 toport 3 (and all power into port 1 to port 2 and there is no port 4).

Unfortunately, with directional couplers there can be a reflection ofthe downstream signal, for instance at the dashed line 124. As aconsequence, the downstream signal is reflected back to port 3 and inpractice this power can be significant or even high compared to thepower of the upstream signal intended to propagated from port 3. Thiscauses a problem in most systems, especially systems that have no meansto deal with the reflection. For this reason filter banks are commonlyused.

FIG. 35B shows an exemplary filter bank 130 which in this example is atriplexer, but could also be a diplexer as further explained below. Inthis figure, three ports 132, 134, and 136, each with its own respectivefrequency band (A, B, C) are provided to filters 133, 135, and 137 eachconfigured to pass a respective predetermined frequency band. Thefilters 133, 135, and 137 are all connected to a common port 138, towhich a coaxial cable going to a subscriber can be connected. Thefilters 133, 135, and 137 are not sensitive to the direction the signalis traveling, but are highly effective at rejecting signals that are notin the frequency band passed by the filters. If a diplexer is usedinstead of the triplexer depicted, the port C does not exist.

The downstream signal amplifier output may be connected to port 134,where the filter 135 passes for instance 54-1200 MHz to the coaxialcable at the common port 138. The upstream signal into the common port(from the coaxial cable) is put out at port A after being filtered bythe filter 133 which, for instance passes signals in the frequency bandof 5-42 MHz. Note that upstream signals outside this frequency band,such as reflections of the downstream signal, are not put out at port A.Thus, this system is robust and applied in practice. In this system,upstream and downstream signals are selected and directed purely on thebasis of filtering the frequency bands they occupy, not based ondirectivity of couplers.

FIG. 35C shows a hybrid split setting element 140 having both a diplexfilter 142 and a directional coupler 144. In this device, thedirectional coupler 144 may receive a downstream signal from amplifier Aand provide it to the diplex filter 142 that passes the frequency bandassociated with the downstream signal (for instance 54-1200 MHz) to thecoaxial cable 146 going to the home. Upstream signals from the home areoutput from the diplex filter 142 to amplifier C in the conventionalupstream frequency band (for instance 5-42 MHz). In addition upstreamsignals from the home in the downstream frequency band (in the case thatthe downstream frequency band is used for bidirectional communication)are output the diplex filter 142 to the directional coupler (orcirculator) and are output at port 3, which provides the upstream signalto amplifier B. The outputs of amplifiers B and C may be summed to asignal D that can be provided to an upstream transmitter. Thus, theupstream transmitter can transmit both the conventional upstreamfrequency band that only contains upstream communication signals and afrequency band that is used for bidirectional communication within thatband. Of course, this band used for bidirectional communication alsocontains unwanted reflected downstream signals and it is assumed thateither the level of these reflections is low enough or the output of thetransmitter is provided to a system component that is able to handle thereflections, for instance using reflection cancellation algorithms indigital signal processing. In this manner, the upstream transmitter canhandle both conventional upstream signals and also upstream signals in afrequency band for using bidirectional communication.

Referring back to FIG. 34A, the depicted single output ONU 100 may beused to customize the split used to service a single home 110, forexample, in a FTTH architecture, using any one of the devices shown inFIGS. 35A-35C. Referring to FIG. 36A, and assuming that thesplit-setting device 112 is a diplexer, the single output ONU mayfabricated to have two alternate ports 112 a and 112 b, each connectedto a respective fixed diplexer integrated into the ONU assembly. Forexample, the port 112 a may be connected to an integrated diplexerconfigured to implement a split having an upstream signal bounded at 204MHz and the downstream signal occupying a frequency band above that, upto 1.2 GHz. Similarly, the port 112 b may be connected to an integrateddiplexer configured to pass upstream signals within the 5-42 MHz range,while passing downstream signals within the 54-1200 MHz range. Thedownstream matching amplifier 106 is connected to each of the ports 112a and 112 b, and similarly the upstream laser driver 108 is connected toeach of the ports 112 a and 112 b, so that regardless of the port usedto connect the home 110 to the desired split, the ONU 100 will operateto implement that split. In this manner, different homes, even withinthe same service group, may be configured to their own unique splits.Furthermore, each home in the service groups can receive a signalpropagated from a common fiber cable 102 because each home is connectedto a diplexer at a selective one of ports 112 a or 112 b that simplyfilters the signals propagated to and from that fiber optic cableaccording to its own split. Those of ordinary skill in the art willappreciate that the number of ports may vary depending on theimplementation, e.g. if it is desired to have options for three or moresplits to be provided to a home.

Whereas FIG. 34A shows an ONU 100 suitable for use in a FTTH deliverysystem that is capable of customizing the split used by each home in aservice group, FIG. 34B shows an alternate ONU 101 suitable for use in,for example, a Fiber-to-the-Curb (FTTC) or Multiple Dwelling Unit (MDU)delivery system. Specifically, the ONU 101 may have multiple ports 114a-114 d, each of which may be used to connect to a respectivelydifferent subscriber 110 a-110 d, at that subscriber's own split,independently of the split used by other subscribers being connected tothe ONU 101. Preferably, because different ones of the subscribers theONU may decide to upgrade over time, and because different ones of thesubscribers may utilize the same split as one or more other subscribersconnected to the ONU 101, the ONU 101 preferably includes receptaclesfor plug-in diplexers rather than having diplexers integrated into theONU. This is illustrated in FIG. 36B.

Referring to FIG. 36B, each port 114 a-114 b may comprise a receptacle(or other suitable interface) into which a diplexer may be selectivelyinserted as needed to configure a subscriber to a particular split asthe need arises. FIG. 36 b, for example, illustrates the port 114 ahaving an diplexer inserted that implements a 2.4 GHz/1.2 GHzdownstream/upstream split, the port 114 b having a diplexer insertedthat implements a 1.2 GHz/204 MHz downstream/upstream split, the port114 c having a diplexer inserted that implements a 1 GHz/85 MHzdownstream/upstream split, and the port 114 d having an diplexerinserted that implements a 750 MHz/42 MHz downstream/upstream split.Thus, as depicted, each of the homes 110 a-110 d shown in FIG. 34B wouldbe configured to receive content at a different split than any of theother four homes connected to the ONU, despite the fact that each homeis within the same service group and that all homes propagate theirupstream and downstream signals over the same fiber optic line 102.Again, the downstream matching amplifier 106 is connected to each of theports 114 a-114 d, and the upstream laser driver 108 is also connectedto each of the ports 114 a-114 d, so that the ONU 100 will operate toimplement the splits of every home.

Over time, however, one or more of the homes 110 a-110 c may want toupgrade service. At that time, the diplexer used to implement the splitto that subscriber would simply be replaced with one implementing thenew split. Those of ordinary skill in the art will appreciate that,although this feature is described with respect to the ONU 101, it couldalso be implemented on the ONU 100 if, for example, a CATV providedwanted to retain the ability to upgrade the ONU 100 to any one of manydifferent splits. In such a circumstance, the ONU 100 may only need oneport 112 rather than ports 112 a and 112 b.

The plug-in diplexer system just described is surprisinglycost-effective. Ordinarily, diplex filters commonly used in node andamplifier systems in CATV networks need to meet strict requirements.First, the amount of suppression of the unwanted frequency band in thediplex filter must be very high in order to obtain a usable upstreamsignal because the output from an amplifier is distributed over manyhomes and therefore has a very high output power level, as well as thefact that the input to the amplifier from upstream signals is very weakbecause there are many homes combined to an amplifier with splitters andcombiners that cause a high loss; therefore, a high suppression isneeded of the downstream frequency band. Second, the loss of the filtermust be very low to avoid both the loss of the high downstream amplifierlevel and degradation of the already weak upstream signal. Third, theresponse of the filter as a function of frequency in the pass-band mustbe very flat because in a CATV system, many amplifiers are cascaded suchthat the signal passes through a large number of diplex filters, and theresponse ripple of the individual filters is therefore magnified.Fourth, amplifiers in the field are high-gain bidirectional amplifiersand diplex filters are used to prevent oscillation of these amplifiers.

In the systems shown in FIGS. 34A-36B, however, none of the foregoingrestrictions are applicable. The downstream and upstream signal levelsare similar at a location close to a home or at the coaxial cable goingto a home, so high suppression of any frequency band is not needed. Thedownstream power level is low and the upstream signal level is high, asmall amount of filter loss can be tolerated. The system contains onlyone diplex filter that the signal must pass through, response ripple isnot compounded so the requirements for the individual filter responseflatness is relaxed. Finally, the system uses O/E conversion, i.e. it isnot a bidirectional RF to RF amplifier configuration, and there is nopotential for oscillation. Because these restrictions on the diplexersmay be relaxed, requiring fewer components, filter tuning may not berequired in manufacturing leading to low manufacturing costs.

Those of ordinary skill in the art will appreciate several features ofthe system shown in FIGS. 34A-36B. First, a DOCSIS scheduler will ensureappropriate functioning of Customer Premises Equipment (CPE) in thesesystems. Furthermore, a 3 dB or even 6 dB reduction in power can beaccommodated by an increase in RF gain, if necessary. Finally, thefeature of using plug-in diplexers can also be utilized inFiber-to-the-Last Amplifier (FTTLA) configurations.

FIGS. 37A and 37B show respective alternate embodiments where the ONUs100 and 101, respectively, are each configured to optionally add a PONpass-through feature to the implementation. This feature may be useful,for example, in circumstances where one or more subscribers connected tothe ONU are devoted to specific PON services such as 10/10 and 10/1 GbpsEPON, or other advanced data services. The PON services may be carriedon different wavelengths on the main fiber trunk 102 and separated outat the BOSA 104 to go to the optional PON pass-through module 120.Specifically, the ONU 100 and the ONU 101 may each include a PONpass-through module 120 that directly interconnects the BidirectionalOptical Subassembly (BOSA) 104 with CPE equipment, such as an OpticalNode Terminal (ONT) using a dedicated fiber connection.

Burst Mode Node

In a fiber deep architecture fiber may be drawn from existing nodelocations to the amplifier locations. The coaxial trunks providing RFsignals to the amplifiers may lose their function, but the distributioncoaxial networks to the homes remain in place. For such a fiber-to-thelast-active system the diplexer requirements may be relaxed becausethese are no longer needed for amplifier isolation, only for bandseparation, resulting in an increase in available bandwidth. Also for afiber deep network, more bandwidth may be offered in general becauseonly one RF active is in the signal chain. Finally, fiber deeparchitecture offers more options to segment traffic. For one node theremay be as many as 32 amplifiers, or even higher; if each amplifier isoperated as a node, then in the reverse direction the optical signalsall need to be received at the original node location, RF added, andre-transmitted. An alternative implementation uses HPON architecture toreceive and combine a large number of optical inputs with little loss innoise performance and bandwidth.

However, in either case, in the event that all the upstream transmittersare active at the same time, the summation of noise due to shot noiseand laser noise of the individual transmitters is large, resulting in anoverall SNR degradation that can be debilitating. In the example case of32 upstream transmitters, for example, it can be as large as10*log(32)=15 dB. In order to prevent such a large degradation,preferably the upstream transmitters are operated in a burst mode,active only if there is upstream traffic generated by users. Thisresembles operation of the upstream transmitter in an ONU that is activeonly when users generate traffic.

The present inventors discovered that it is advantageous to locate theupstream burst mode functionality of an ONU (that is normally located atthe customer premises) at a location higher in the network, such as anexisting amplifier location. The amplifier may be upgraded by re-placingthe lid with an ONU functionality that is coupled to the amplifierforward and reverse gain module. This provides a relatively low costmethod to upgrade a network to fiber-deep operation with enhancedbandwidth capabilities and as a stepping stone to driving fiber evendeeper such as FTTC. It should be noted that as fiber is driven deeper,the number of reverse transmitters increases and the need to keepaccumulated transmitter noise under control by operating transmitters inburst mode becomes more urgent.

FIG. 38 shows an amplifier upgrade 200 with an ONU “lid.” In this systema coaxial cable 202 is connected to the input of system 200 and providesan RF signal in the 54-860 MHz range. Due to losses in the precedingcoaxial network, the RF level is low. The downstream 54-860 MHz signalis separated by a diplex filter 204 and provided to a forward amplifier206. which outputs the forward signal at a high level. The output ofthis amplifier 206 represents the downstream signal that is provided toa second diplex filter 208 that outputs that signal to an output coaxialcable 203 at high level which is sufficient for distribution to homes.In the upstream direction, modems at the customer premises send signalsupstream that arrive at the output diplex filter 208 of the amplifierupgrade 200. The diplex filter 208 separates these upstream signals, forinstance in the 5-42 MHz range and provides them to the reverseamplifier 207. The reverse amplifier 207 outputs these signals at highlevel to the diplex filter 204, which in turn outputs these signals inupstream direction on the incoming coax cable. The level is high enoughto overcome losses in the upstream signal.

Referring to FIG. 39, the amplifier upgrade 210 may be modified to havean ONU functionality. In this figure, an optical fiber 212 is connectedto the input of the ONU 214 and can provide an RF signal in the 54-5000Hz range for instance at 1550 nm. The downstream signal or a bandwidthselection thereof is directly provided to the forward amplifier 216outputting the forward signal at a high level. The output of thisamplifier 214 represents the downstream signal that is provided to thediplex filter 218 that outputs that signal to an output coaxial cable213 at high level, which is sufficient for distribution to homes. In theupstream direction modems at the customer premises send signals upstreamthat arrive at the diplex filter 218. The diplex filter 218 separatesthese upstream signals, for instance in the 5-204 MHz range and providesthem to the reverse amplifier 217. The reverse amplifier 217 providesthe signals at high level to the ONU 214. The ONU 214 outputs an opticalsignal modulated with the RF information for instance at 1610 nm.

Compared to the normal location of an ONU at the customer premises, thesystem shown in FIG. 39 has several advantages. First, the output levelfrom the ONU can be low because the forward amplifier provides highgain. Also the input level to the ONU is high due to the presence of thereverse amplifier. Thus it becomes clear that when an ONU is placedinside the coaxial distribution network there will generally be excessgain and the ONU may be simplified to operate with low output level andhigh input levels.

An existing RF amplifier typically comprises three sections. Apre-amplifier stage for the downstream signal, a post amplifier stage ofthe downstream signal and an upstream signal amplification stage. Whenmigrating to the disclosed architectures, in some embodiments thepreamplifier stage and the reverse amplifier stage would be replacedwith an ONU. A typical ONU may have very low dissipation and with theburst mode operation, limit the power further. In fact the ONUdissipation may be greatly reduced such that an existing amplifierhousing may support the ONU operation with merely a lid upgrade and nosignificant increase of power usage. Furthermore, there are multipleavenues to amplify and/or attenuate the optical and RF levels to presentto the CMTS port. This can be accomplished via an RF OMI adjustalgorithm.

In an ONU, if implemented with high and low bias states (or even withthe laser on all the time) rather than a laser off state, noise injectedinto the ONU below the turn-on threshold of the ONU would still beamplified and provided to the active laser such that this noise reachesthe active combiner. Thus in such an ONU there is a need to reduce thegain for RF input below the turn-on threshold of the ONU. Controlsystems such as laser bias control, amplifier bias control and gaincontrol can be used together, independently or in any combination toreduce that noise. Without loss of generality these techniques may beused for analog or for digital applications such as DOCISIS 3.0, 3.1, 1G/10 G EPON or 1 G/10 G PON.

It is also known that the there is a finite turn-on time for the ONU.For example for RFoG, the turn on time should preferably be between 100ns thru 1000 ns (or 1 us). A turn on time that is very fast creates avery high low frequency noise that decreases over the frequency range.Unfortunately, most of the currently deployable upstream signals aredegraded by this low frequency noise phenomenon, which extends to 50 Mhzand beyond. To make matters worse, this noise is spiky in that theinstantaneous noise burst could be much higher than what is commonlyseen on a spectrum analyzer with moderate video bandwidth.

FIG. 40 shows a typical ONU upstream implementation, which generallyillustrates a system 220 where an RF detector 222 detects whether an RFsignal is present at its input 224. If a signal is detected, the RFdetector 222 passes the signal through to an amplifier 226 and alsosignals a laser bias control module 228 to turn on at time t0 a laser227, which has a turn-on time 229. The amplifier 226 amplifies the RFsignal that is passed through from the RF detector circuit 222. Theamplified signal drives the laser 227. The laser's output is propagatedfrom the ONU on a fiber 221. The turn-on time 229 of the laser has aprofound effect on the spectrum produced by the turn-on event

FIGS. 41A and 41B show estimated spectra for a rise time of 100 ns and 1us, respectively, with a typical intended signal at 40 MHz. For a shortrise time the noise due to the ONU turn-on is of the same order ofmagnitude as the intended signal. With a slower laser turn on thiseffect can be mitigated. If there is just one ONU on at any given pointin time, the effect of low frequency noise due to ONU turn-on is notfelt, because the DOCSIS load is inset after the laser has fully turnedon. However, when there are multiple ONUs that can turn on at any givenpoint in time, the situation becomes more complex.

If there was an ONU already on, and another ONU turns on while the firstone is transmitting data, then the high noise spikes—describedabove—occur across the frequency spectrum. Depending upon the relativeRF levels of the signals and the noise spikes, the signal may experiencepre- or even post-FEC errors (when measured at the CMTS for example).This issue becomes more and more pronounced as the numbers of ONUs thatcan turn on is increased, as is likely to happen with DOCSIS 3.1. Whilethis effect has always existed, it is only apparent as a residual errorfloor when the OBI and its induced errors have been eliminated.

An additional impairment is caused by the application of the RF signalbefore the laser has fully turned on and has stabilized. This can occurfor example if the laser turn-on time is slower than the DOCSISPreamble, which may be applied before the laser has reached steadystate. In this case, the DOCSIS Preamble is a QPSK signal and can behigher than the regular RF signal to follow (for example it may be 6 to10 dB higher depending upon the conditions). In this case, the laser isover-driven while still in a low power state and experiences very largeclipping events that will spread throughout the RF spectrum and degradeother signals that may exist at the same time. While this effect alwaysoccurs, it is more obvious with the elimination of the OBI, since theOBI induced errors are removed from the system.

FIG. 42 illustrates this problem showing a bias around which a laser ismodulated with a sine wave signal. During the time that the laser biasis insufficient the output signal is clipped. For slower laser turn-onthe duration of the clipping is increased. While one may want to reducethe low frequency RF spikes across the board by having a slower turn ontime, the higher clipping effects described above may limit those gains.This disclosure provides an innovative approach to take both of theseeffects into account.

Referring to FIG. 43, a system 230 includes an RF detector 232 thatdetects whether an RF signal is present at its input. If a signal isdetected, the RF detector 232 passes the signal through to an amplifier234 and also signals a laser bias control module 236 to turn on at timet0 a laser 237, which has a turn-on time 238. The laser bias controlmodule 236 preferably modulates the bias of the laser 237 to achieve afull turn-on of the laser 237 over a turn-on time 238 that is preferablyas slow as possible, e.g. the slowest turn-on time allowed by the RFoGstandard, or in some embodiments even longer. Preferably, the laser biascontrol module 236 will ramp up the RF gain in proportion to the laserturn on, or delay the RF turn on until the laser is sufficiently turnedon to experience negligible laser clipping. This may be done for exampleif the RF amplifier gain is adjusted in proportion to the laser bias,thus fundamentally preventing the over shoot and clipping events. Withan RF gain factor proportional to the laser bias the clipping no longeroccurs as shown in the FIG. 44. In some embodiments, the turn-on time ofthe laser 237 could be up to 500 ns, or longer. This may greatly reducethe low frequency noise. The turn-on time for the laser may be linear,as shown in FIG. 43, or may implement a transition along any otherdesired curve, such as a polynomial curve, an exponential curve, alogarithmic curve, or any other desired response

However the variation in RF level during the laser turn-on maypotentially cause an issue in the burst receiver that may expect a nearconstant RF level during the laser turn-on. In case that is required,the RF turn-on may be delayed and apply a faster time constant than theoptical power turn on. This is illustrated in FIG. 45. FIG. 46illustrates a ONU that implements such a delay, and the most importantsignals therein. Specifically, a novel ONU upstream architecture 240includes an RF detector 242 that detects whether an RF signal is presentat its input. If a signal is detected, the RF detector 242 passes thesignal through to an amplifier 244 and also signals a laser/amplifierbias control module 246 to turn on at time t0 a laser 247, which has aturn-on time 248. The laser/amplifier bias control module 246 preferablymodulates the bias of the laser 247 to achieve a full turn-on of thelaser 247 over a turn-on time 248 that is preferably as slow aspossible, e.g. the slowest turn-on time allowed by the RFoG standard, orin some embodiments even longer. In some embodiments, the turn-on timeof the laser 247 could be up to 500 ns, or longer. This may greatlyreduce the low frequency noise. The turn-on time for the laser may belinear, as shown in FIG. 45, or may implement a transition along anyother desired curve, such as a polynomial curve, an exponential curve, alogarithmic curve, or any other desired response.

The amplifier 244 amplifies the RF signal that is passed through fromthe RF detector 242. The amplified signal drives the laser 247.Preferably, when amplifying the RF signal from the RF detector 242, thelaser/amplifier bias control module 246 includes a circuit thatmodulates the amplifier gain to be proportional to the laser bias, butwith a delay 249 relative to the time t₀ that the laser 247 begins toturn on. In other words, the bias control module 246 detects an RF inputsignal at t0 and turns on the laser slowly in a time t_on_1. At a secondtime t1, optionally delayed with respect to t0, it starts to turn on theRF gain with a rise time t_on_gain. Preferably, the rise time of theamplifier gain is faster than the rise time of the laser turn-on. Insome embodiments, the laser/amplifier bias control module 246 simplyswitches on the RF gain, i.e. t_on_gain is set to a very short value.

The ability to simultaneously slow the laser turn on and to provide anRF gain to the input in proportion to the laser turn on, or delayed withrespect to the laser turn on is an innovative feature that has greatpotential in all applications, and without loss of generality thesetechniques may be used for any analog application such as DOCISIS 3.0 orDOCSIS 3.1.

There may be benefits, at times, where/when these “modified ONU-amps”,or mini-nodes are better used in the continuous mode. Therefore,preferably the burst mode operation is selectable or controls itselfautomatically or can be remotely controlled in the following manner.First, the modified amp/mini node is operated in the burst mode bydefault. Second, a sensing circuit is included in the unit, which willswitch into “always on” mode. In a first embodiment, the switch canhappen on a “trigger sequence” received from a downstream pattern,injected into the service group traffic of a given CMTS. In a secondembodiment, the switch can happen on a “trigger sequence” coming from anupstream pattern. In a third embodiment, the switch may be triggered bystrong upstream traffic; above a certain percentage of detected upstreamtraffic, such as 75%, the switch may trigger to an “on” state and if itdrops below a certain percentage, such as 50%, it may drop back to burstmode operation. In a fourth embodiment, the device may operate with afast turn-on when upstream traffic is initiated combined with a slowturn-off, both in terms of turn-off delay and in terms of turn-offtransition time. Preferably, the node is also capable to operate thelaser in continuous mode while still operating the RF switch such thatinput noise to the node is suppressed when the RF switch is off duringtimes that no upstream signal to transmit is provided to the node.

Full Duplex Bidirectional Transmission

HFC to Fiber to the Last Active to Fiber (FTLA) to Fiber to the Curb(FTTC) are all different way stations along the way to (Fiber to theHome) FTTH networks. Due to the finite build capacity, it will likelytake decades to effectively migrate all networks to fiber to the home.Therefore, it becomes more and more attractive to be able to maximizecapacity over systems that are Fiber Deep (FD) such as FTLA and FTTC asthe networks migrate towards FTTH.

While this may be accomplished by D3.1 in combination with othertechniques such as spectrum enhancement, the issue of upstreamtransmission remains complicated. Today bi-directionality on coax cableis effected by means of diplex filters, such that non-overlapping lowerfrequency band (5-204 MHz for example) is used as the upstream and thehigher frequency (258 MHz to 1218 MHz) band is used for the downstreamtransmission.

Since total bandwidth in general is limited, an increase in the upstreambandwidth necessarily limits downstream capacity. However as networks gofiber deep, the upper bandwidth can be increased significantly. Forexample, the present inventors have discovered that an effectivebandwidth is upwards of 2.5 GHz. However, note that increasing upstreambandwidth even in this case, takes away from the downstream bandwidth,and furthermore, as the frequency split becomes higher and higher, moreand more spectrum is wasted in effecting proper filtering needed toisolate forward and reverse signals.

These are resolved in FTTH systems; Since duplexing is doneoptically—there being two wavelengths, one for the forward direction(1550 nm for example) and one for the reverse direction (1610 nm forexample)—there can be completely overlapping forward and reversesignals, thus increasing capacity in both directions. Furthermore, theability to increase capacity in this way also has the major benefit ofbeing able to support existing CPEs that typically have bandwidths asindicated earlier.

The inability of having full duplex bi-directional transport of signalsover coaxial cable is therefore serious limitation, one that limitscapacity. Though one way to overcome this limitation is by migrating toFTTH, but that is an expensive and time consuming activity due tolimited build capacity at most MSOs.

Disclosed is an elegant way of achieving full-duplex bi-directionaltransmission (FDB) in FTTC. Generally, FDB cannot happen in coax becauseof the presence of significant reflections. And unlike opticalattenuators, RF attenuators do not have sufficient isolation to preventreflections and preserve isolation. Therefore, it is paramount tomitigate reflections, enhance isolation. In addition, there is a need tohave meaningful echo cancellation in the system to clear out theresidual reflections.

In this solution, since it supposes FDB on FTTC systems, there is merelya piece of coax roughly 150 ft to 200 ft between an ONU (optical networkunit), split between 4 to 8 ways and between 2 to 4 subscribers. Therecould be other combinations as well. The disclosed solution appliesactively amplified splitters on each splitter leg in the ONU, thuseffectively preventing large reflections from the ports of the ONU.Furthermore, the CPE is now able to know when and what it has lasttransmitted. Even if stray reflections arrive at it, it is able to echocancel these. Similar, the CMTS will be able to echo cancel reflections.

As an example, if the downstream is 100 MHz to 2.5 GHz wide and theupstream is 5 MHz to 1.2 GHz wide, the downstream transmission isgenerally not burst mode and is continuously available to all the ONUsand to all the CPEs. The upstream transmission is in burst mode and isavailable only when the CPE has an RF that needs to reach the ONU. Sincea large number of subscribers subscribe to popular tiers, these are alllikely to be below 42 MHz, 85 MHz or 204 MHz as the case may be. Thereare very few subscribers in general who would subscribe to the very hightiers that would require 1.2 GHz upstream (these would be the onestaking 10 Gbps tier in the US). These subscribers will have high endCPEs that will have echo cancellation. During most of the time, theupstream is below a set limit (as indicated above 204, 85 or 42 MHz) andthe upstream is above it, and for most of the subscribers owningstandard equipment, there is no need to be affected, one way or another.The CMTS behaves normally also, since the RF frequencies are naturallysegregated.

When a premium customer bursts, in the upstream, the signal travels tothe ONU; due to the active port, it reflects very little to itself andto other subscribers in its vicinity thus not affecting the performancein any meaningful way. This signal travels to the CMTS. At the CMTS, theport accepts upstream signal; there the signal is digitized and sent toprocessing into the CMTS core. If there is a reflection of thedownstream signal that is sent along with the upstream signal due tofinite reflection from the CPE, it would be echo cancelled at the CMTS.Thus, the entire process of echo cancellation is now subjected only torather limited echo cancellation requirements with this novel design.

FIG. 47 shows a multiport ONU 300 connected to user cable modems (CM)302 via drop cables 303 to the left and to an upstream device (orheadend) via fiber 304 to the right. Those of ordinary skill in the artwill appreciate that a cable modem may also represent a home gatewaydevice, for instance high end customers may have a gateway at the home.

The cable modems are differentiated between hi-user cable modems thatuse extended spectrum operation and lo-user cable modems that only usethe standard spectrum range. The downstream signals with a power P_fwdto the cable modems are indicates with arrows 306, where solid arrowsrepresent intended downstream signals and dashed arrows representunintended reflections with power R_fwd of the downstream signals.Similarly, the upstream signals with power P_rev are indicated witharrows 308, where the intended signal is solid and the unintendedreflections of upstream signals with power R_rev are indicated withdashed lines. On the drop cables 303, the arrows representing thesignals could be interpreted as an amount of spectrum covered by thesignals, see for instance the second cable modem connection from the topthat has a frequency axis drawn under the drop cable such that thearrows are “mapped” to that frequency axis such that the upstreamsignals cover 5-100 MHz and the downstream signals cover 100-1200 MHz.For low-end users the spectra of upstream and downstream signals do notoverlap and therefore upstream and downstream signals do not interfere.For the high end user however the upstream signal can use an extendedbandwidth range, partially overlapping with the upstream signal.

The drops 303 are connected to an active ONU that includes isolationamplifiers 310 and 312 to drive forward signals to the cable modems andamplify upstream signals received from the cable modems. The downstreamisolation amplifiers 310 strongly attenuate upstream signals such thatthese do not reach the 1:N splitter 314 used to distribute the forwardsignals over the ports. This relaxes the isolation requirement for the1:N splitter 314; if this splitter had limited isolation then upstreamsignals from a first port could be put out at a second port and if theupstream signal would overlap with the downstream signal spectrum forthat port it would cause interference. However, through the use ofisolation amplifiers this problem is avoided.

A directional coupler 316 with loss of cplr_dB is used to provideupstream signals to a reverse isolation amplifier 312 that puts outsignals to a 1:N combiner via an optional switchable filter. Theamplifier 312 is provided enough isolation such that the reverse signalsof the different ports are isolated from each other. Finally thedownstream signal splitter 314 is connected to a downstream signalreceiver 320 and the upstream signal combiner 318 is connected to anupstream transmitter 322. The receiver 320 and transmitter 322 areconnected to a fiber link 324 that goes upstream, generally via a WDM.

In this architecture the reflected downstream signal will pass throughthe isolation amplifier 312 and is combined on the 1:N combiner 318. Theoptional filters 326 may be used to limit the bandwidth of the upstreamsignal such that the reflected downstream signals are rejected at lowfrequencies; that is the frequency range used by most low-end users. Fora high end user that uses an extended frequency band however such afilter 326 would need to be turned off or set to another appropriatebandwidth as will be discussed later. The filter 326 could be controlledfrom a head end site. This implies that some of the downstream signalwill be reflected and the spectrum of the reflection can (in part)overlap with the spectrum of upstream signals that are used by ahigh-end user. A high end user may use spectrum up to 1200 MHz or evenbeyond.

A reflection of the upstream signal, for instance from the multiport ONUto the cable modem of the high end user, causes problems for downstreamsignal reception by the high end user when the spectrum of the upstreamsignal overlaps with that of the downstream signals. Note that thiscauses a problem only for the high end user due to the use of isolationamplifiers 312 in the multiport ONU 300.

Thus, there are two reflections that cause problems in this type ofsystem, both affecting a high end user only. Firstly a reflection of thedownstream signal ends up in the upstream signal and is transmitter tothe CMTS in the head end. Secondly a reflection of the upstream signalcan overlap with the spectrum of the downstream signal at the high-enduser cable modem. In both cases echo cancellation may be used tocompensate for the interference caused by these reflections. The CMTSmay correlate the reflected downstream signal received at an upstreamport to the downstream signals that were sent by that CMTS and use thisto compensate a signal received at an upstream port such that (in thedigital domain) the reflected downstream signal components are cancelledout. The cable modem may correlate the reflected upstream signal tosignals it transmitted and use this to compensate a signal received inthe downstream direction such that (in the digital domain) the reflectedupstream signal components are cancelled out. This may be a relativelystraightforward task for a cable modem sharing upstream and downstreamports or with those ports in close proximity and with reflections thathave a delay not more than the roundtrip time of the drop cable. For theCMTS however in many instances upstream and downstream ports may not beshared and the roundtrip delay for the reflections to be cancelled canequate to more than 100 km of transmission line length. Therefore,reflection cancellation at the CMTS side may be challenging.

Since the downstream signal reflection towards the CMTS is expected tobe the most difficult to handle, this reflection is discussed first. Atthe outset, for existing connections for average users (up to 99% ofactual users) overlapping spectrum may not be used. The addition of aswitchable filter in the upstream path as shown in FIG. 47 permitselimination of reflected downstream spectrum for most customers, butdoes require a control path for that filter, such as control by the headend or control via an upstream control path from the cable modem or homegateway. Furthermore, the switchable filter may also include RF on/offswitching; upstream signal detection is needed in any event to controlthe upstream laser transmission of the multiport ONU. In case upstreamsignals are detected per port, then reflected downstream signals caneffectively be suppressed for ports without upstream transmissions.Reflections in the plant can be expected to be on the order of −20 dB.In case an SNR of 40 dB (due to reflection) is targeted, then a leveldifferential of 20 dB between upstream and downstream signals may beappropriate such that combined with −20 dB of reflected signal level,the wanted to unwanted signal ratio is 40 dB. This means that in FIG. 47P_rev may be set 20 dB higher than P_fwd. This also ensures thatupstream signal transmission may be detected with a good discriminationfrom downstream signal transmissions to control any optional RF switchesthat may be present. Of course, a drawback to this approach may be thatthe reflected upstream signal to the cable modem may then be of similarmagnitude as the downstream signal to the cable modem, complicating theecho cancellation of the cable modem that would need to attain 40 dB ofreflection cancellation or more.

The SNR required for upstream transmission may be deliberately limitedin exchange for more bandwidth; for every 3 dB of reduction in SNR thesame transmission power can provide a doubling in available bandwidthsuch that the total throughput may be the same or higher. For instancein case an SNR of 24 dB would be sufficient for the upstream signal thenthe upstream signal level may not be raised much above the downstreamsignal level facilitating easier reflection cancellation at the modem.

It should also be considered that for moderate reverse spectrum growthto for instance, a 204 MHz split the high-end user that utilizes thisspectrum may not require the 108-204 MHz downstream spectrum and thecable modem or home gateway may simply discard that downstream spectrumand transmit upstream with sufficient power to provide enough SNR forthe upstream spectrum.

The drop cable can support spectrum to very high frequencies such as 6GHz; another alternative places the upstream signal spectrum in afrequency range not used for downstream signals; statically ordynamically. The upstream transmitter in the multiport node may bebandwidth limited such that it may include a down-converter to placehigh frequency upstream signal spectrum in a lower frequency band thatmay overlap with the downstream signal spectrum; once in the opticaldomain the upstream and downstream signals are separated and nocrosstalk or reflection of one into the other can occur. Alternatively,high frequency upstream signals may be limited to one octave such thatsecond order distortions from the upstream link can be ignored; suchdistortions generally occur when using a low cost directly modulatedlaser in the upstream direction unless a low dispersion window of thefiber is used. In such a scheme the 5-100 MHz upstream signal may beup-converted to a high frequency, for instance 2 GHz such that the cablemodem can complement that signal with further signals in the 2-4 GHzrange. The 2-4 GHz range may then be allocated by the CMTS in ahalf-duplex mode such that high peak up- and downstream rates up toseveral 10 Gbs can be achieved without reflection cancellation methods.At the receiver side the 5-100 MHz signals would be down-converted toconnect to regular CMTS ports while the high frequency signals may passdirectly to extended bandwidth CMTS ports. This should suffice for thenext couple of decades of bandwidth growth.

Finally, it should be considered that ultimately a small fraction ofcustomers may need extremely high peak rate transmissions with fullduplex capability (such as 40 Gbps downstream and 20 Gbps upstreamsimultaneously). At that time, for such customers a fiber may beprovided to the home in this architecture or a Siamese cable including acoaxial connection and a fiber connection or with two coaxialconnections. When this is provided, the upstream traffic can beseparated from the downstream traffic and full duplex bidirectionaloperation is enabled. Such connections can be added within the proposedarchitecture that already offers both half duplex operation for highpeak rates and conventional DOCSIS operation for normal peak rates.

Therefore, in some embodiments, the complexity of echo cancellation maynot be necessary so long as a flexible allocation of upstream anddownstream transmit frequencies is permitted. Diplex filters shouldgenerally be avoided to ensure this flexibility. Upstream transmissionlevels should generally be set high enough that reflected downstreamsignals do not significantly alter the power budget of the upstreamtransmitter. Reflected upstream signals are limited to the drop cablesof individual users and, due to the multiport ONU design with isolationamplifiers these do not propagate further in the system such that thereis a high tolerance to such reflections.

Up until now, bidirectional transmission in the same frequency band wasnot considered in CATV networks, in favor of the use of diplex filtersthat are needed to provide isolation in CATV amplifier chains. FTTHenables overlapping FDB transmission and therefore enabling FDB hasdirect and specific benefit in FTTC environment thus increasingavailable capacity and decreasing overall build cost and improvingtimely response to capacity needs.

Use of 10 Gbps in G.Fast twisted pair is only eliminates crosstalk,which is not equivalent to eliminating reflections. On CAT6 cableshowever, 10 GbE implementations do provide both crosstalk and reflectioncancellation, illustrating that such techniques are cost effective

FIG. 48 illustrates an exemplary cable system 400 that may implement allthe systems and methods disclosed in the present application. The system400 includes a head end facility (HEF) 410, a plurality of hubs420(1)-420(m), and associated with each hub, a plurality of nodes430(1)-430(n) and a plurality of customers 460(1)-460(p). The HEF 410 orhubs 420 may have a cable modem termination system (CMTS). Each of thenodes 430 has one or more corresponding access points, and each of thecustomers 460 has one or more corresponding network elements 462, shownin FIG. 48 as a cable modem.

A single node 430 may be connected to hundreds of network elements.Described herein are techniques related to a cable modem network element462; however it should be understood that the cable modem is used by wayof example as the concepts apply to other network elements. Examples ofnetwork elements include cable modems (as shown in FIG. 4), set topboxes, televisions equipped with set top boxes, data over cable serviceinterface specification (DOCSIS) terminal devices, media terminaladapters (MTA), and the like. Thus, where reference is made to a cablemodem, the concepts also apply more broadly to a network element.

A cable system 400 provides one or more of commercial TV services,Internet data services, and voice services, e.g., Voice-over-InternetProtocol (VoIP) to one or more customer locations 460 (i.e., end users)in a given geographic area. To provide these services, the HEF 410 inthe example cable system 400 in FIG. 48 is shown coupled via a contentdelivery network 415 to a plurality of content providers 405, an IPTVserver 416, and a public switched telephone network (PSTN) 417.

The content delivery network 415 may be a cable data network such as anall coaxial or a hybrid-fiber/coax (HFC) network. Of course, otherbroadband access networks such as xDSL (e.g., ADSL, ADLS2, ADSL2+, VDSL,and VDSL2) and satellite systems may also be employed. In embodiments,the content delivery network 415 comprises, for example, apacket-switched network that is capable of delivering IP packets from anIPTV Server 416 to clients 460(1)-460(p), using, for example, a cabledata network, PON, or the like. Examples of a content delivery network415 include networks comprising, for example, managed origin and edgeservers or edge cache/streaming servers.

The content delivery servers 415 deliver content via one or more wiredand/or wireless telecommunication networks to users 460(1)-460(p). In anillustrative example, content delivery network 415 comprisescommunication links 450 connecting each distribution node and/or contentdelivery server to one or more client devices, e.g., for exchanging datawith and delivering content downstream to the connected client devices460(1)-460(p). The communication links may include, for example, atransmission medium such as an optical fiber, a coaxial cable, or othersuitable transmission media or wireless telecommunications.

FIG. 48 illustrates a hybrid fiber-coaxial (HFC) cable network system400. A typical HFC network uses optical fiber for communications betweenthe headend and the nodes and coaxial cable for communications betweenthe nodes and the end user network elements. Downstream (also referredto as forward path) optical communications over the optical fiber aretypically converted at the nodes to RF communications for transmissionover the coaxial cable. Conversely, upstream (also referred to as returnpath) RF communications from the network elements are provided over thecoaxial cables and are typically converted at the nodes to opticalcommunications for transmission over the optical fiber. The return pathoptical link (the optical components in the HFC network, e.g. thetransmission lasers, optical receivers, and optical fibers) contributeto the performance of the HFC network. In this HFC network exampleembodiment, the nodes 430 communicate via optical fibers with the hubs420 and via coaxial cable to customer premises 460.

The HEF 410 and/or the hubs 420 may be coupled to the IPTV server 416and PSTN 417 via CDN 415, e.g., the Internet, for providing Internet andtelephony services (e.g., to and from customer 460(1)-460(p)) via theCMTS. The CMTS 425, in an embodiment, is a general-purpose computingdevice or application-specific integrated circuit (ASIC) that convertsdownstream digital data to a modulated RF signal, which is carried overthe fiber and coaxial lines in the HFC network 450 to one or morecustomer locations 460. A communication interface may connect the CMTS425 to the content delivery network 415 for routing traffic between theHFC network 450 and the internet network, the IP network 415, a PSTN,and/or the content providers 405. The various content providers, 405 forexample, may be the source of media content (e.g., movies, televisionchannels, etc.).

It should be noted that there are multiple embodiments of a CMTSarchitecture, such as a CMTS with an integrated physical (PHY) layer, aCMTS with a distributed PHY, or a Converged Cable Access Platform (CCAP)architecture in which the QAM is placed in an edge QAM. In FIG. 4, theedge QAM 412 is shown in the headend, but the edge QAM 412 may belocated downstream from the CMTS 425. The CMTS 425 may host downstreamand upstream ports and may use separate F connectors for downstream andfor upstream communication for flexibility. In embodiments, acommunication interface utilizing downstream channels 1-4 connects theCMTS 425 to a portion of the HFC network 450 for communicating over theHFC network 450.

By way of example, embodiments below describe a cable modem networkelement at the customer's premises for receipt of the modulated signalsfrom the HEF and/or CMTS. A cable modem is a type of network bridge andmodem that provides bi-directional data communication via radiofrequency channels on a cable network, such as a hybrid fiber-coaxialplant (HFC) or RFoG infrastructure. For example, a cable modem can beadded to or integrated with a set-top box that provides a TV set withchannels for Internet access. Cable modems may deliver broadbandInternet access in the form of cable Internet, taking advantage of thehigh bandwidth of an HFC or RFoG network. Cable modems can also delivervideo services using Internet Protocol (IP). For example, the cablemodem 462 may be connected to IPTV receivers or other items of CPE. Acustomer PC or laptop as well as other associated devices such astablets, smartphones or home routers are termed customer premisesequipment (CPE).

The network element, e.g., cable modem, 462 is connected through thenetwork 450 to the CMTS 425. The cable modem converts signals receivedfrom the CMTS 425 carried over fiber and/or coaxial lines in thenetwork. Cable modems 462 convert the digital data to a modulated RFsignal for upstream transmission and convert downstream RF signal todigital form. Thus, the conversion is done at a subscriber's facility.The cable modem 462 demodulates the downstream RF signal and feeds thedigital data to a CPE or an IPTV, for example. On the return path,digital data is fed to the cable modem (from an associated PC in theCPE, for example), which converts it to a modulated RF signal. Once theCMTS 425 receives the upstream RF signal, it demodulates it andtransmits the digital data to its eventual destination. Cable modems 462are therefore useful in transforming the cable system into a provider ofvideo, voice and data telecommunications services to users.

The cable network 400 may implement the disclosed load balancingtechniques using a Data Over Cable Service Interface Specification(DOCSIS) protocol. DOCSIS is an international telecommunicationsstandard that permits the addition of high speed data transfer to anexisting cable television (CATV) network, such as cable network 400. TheDOCSIS protocol is the protocol used to send digital video and databetween a hub or headend facility and cable modem. DOCSIS is used toconvey Internet or other packet based networking information, as well aspacketized digital video between CMTSs and CMs. DOCSIS is employed bymany cable operators to provide Internet access over their existingnetwork infrastructures, e.g., hybrid fiber-coaxial (HFC)infrastructure, PON architectures, etc. While embodiments are disclosedwith reference to DOCSIS, the load balancing implementations may applyto other networks or systems.

A typical DOCSIS architecture includes a cable modem (CM) located at thecustomer premises, and a cable modem termination system (CMTS) locatedat the CATV headend, as in the example cable network 400 depicted inFIG. ______. In an embodiment, a memory in the headend, such a memory ofthe CMTS 430 or edge device, may include a DOCSIS program thatimplements the DOCSIS specification.

DOCSIS provides a variety in options available at Open SystemsInterconnection (OSI) layers 1 and 2, the physical layer, and the datalink layer. A DOCSIS physical layer may include the basic networkinghardware transmission technologies of a network. A DOCSIS physical layerdefines the means of transmitting raw bits rather than logical datapackets over a physical link connecting network nodes. The bit streammay be grouped into code words or symbols and converted to a physicalsignal that is transmitted over a hardware transmission medium. Themodulation scheme to use and similar low-level parameters are defined bythe DOCSIS scheme.

In one or more examples, the functions described herein may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium and executed by a hardware-based processingunit. Computer-readable media may include computer-readable storagemedia, which corresponds to a tangible medium such as data storagemedia, or communication media including any medium that facilitatestransfer of a computer program from one place to another, e.g.,according to a communication protocol. In this manner, computer-readablemedia generally may correspond to (1) tangible computer-readable storagemedia which is non-transitory or (2) a communication medium such as asignal or carrier wave. Data storage media may be any available mediathat can be accessed by one or more computers or one or more processorsto retrieve instructions, code and/or data structures for implementationof the techniques described in this disclosure. A computer programproduct may include a computer-readable medium.

As described with respect to FIG. 48, a communication interface mayconnect the edge device 440 to the IP network 420 and HFC network 450. Abus is a communication medium that may connect a processor, e.g., CMTSprocessor, a data storage device, communication interface, DOCSISexternal physical interface (DEPI), and memory (such as Random AccessMemory (RAM), Dynamic RAM (DRAM), non-volatile computer memory, flashmemory, or the like). In embodiments the communication interfaceutilizes downstream channels (e.g., channels 5-8) to communicate withthe HFC network 450. The DEPI may connect the edge device 440 to theCMTS 430. In embodiments, on the edge device 440 is anapplication-specific integrated circuit (ASIC).

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and blu-ray disc wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

In an embodiment, a computer-readable storage medium has stored thereoninstructions that, when executed, cause a processor to communicate witha cable modem via a first channel or a first channel set; transmit, froma cable modem termination system (CMTS), a change request directingmovement of a cable modem to a second channel or a second channel set,wherein the change request indicates an initialization technique for thecable modem to perform once synchronized to the second channel or thesecond channel set; determine whether the initialization technique wasperformed successfully by the cable modem; and transmit a retry requestindicating an initialization technique if it is determined that theinitialization technique in the change request was unsuccessfullyperformed by the cable modem, wherein the CMTS is configured toiteratively transmit subsequent retry requests based on a list ofinitialization techniques.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablelogic arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the term “processor,” as used herein may referto any of the foregoing structure or any other structure suitable forimplementation of the techniques described herein. In addition, in someaspects, the functionality described herein may be provided withindedicated hardware and/or software modules configured for encoding anddecoding, or incorporated in a combined codec. Also, the techniquescould be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinter-operative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

The terms and expressions that have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention in the use of such terms andexpressions of excluding equivalents of the features shown and describedor portions thereof, it being recognized that the scope of the inventionis defined and limited only by the claims that follow.

1. An Optical Network Unit (ONU) comprising a first input receiving adownstream optical signal from a head end and a plurality of highisolation ports each receiving a respective upstream RF coaxial signalfrom, and transmitting a respective downstream RF coaxial signal to, arespective one of a plurality of subscribers, each high isolation portcapable of attenuating RF reflections.
 2. The ONU of claim 1 having anRF splitter and an active amplifier interposed between the RF splitterand each high isolation port, the active amplifier amplifying signals inthe downstream direction.
 3. The ONU of claim 2 including a directionalcoupler between each active amplifier and its associated high isolationport.
 4. The ONU of claim 1 including a combiner and a switched filterbetween the combiner and the splitter.
 5. The ONU of claim 4 where thehigh isolation port where the switched filters reject reflections at lowfrequencies.
 6. A system comprising: a head end; an Optical Network Unit(ONU) comprising a first input receiving a downstream optical signalfrom the head end and a plurality of ports each receiving a respectiveupstream RF coaxial signal from, and transmitting a respectivedownstream RF coaxial signal to, a respective one of a plurality ofsubscribers; and a plurality of cable modems each connected to arespective one of the plurality of ports, where the ONU attenuatesreflections of the downstream signal from at least one of the pluralityof cable modems.
 7. The system of claim 6 where at least one cable modemecho cancels reflections.
 8. The system of claim 6 where the CMTS echocancels reflections.
 9. The system of claim 6 where at least one cablemodem discards a range of frequencies in the downstream signal and usesthat range to transmit an upstream signal.
 10. The system of claim 9where the range of frequencies is between 100-200 MHz.
 11. The system ofclaim 6 where at least one cable modem transmits an upstream signal at afrequency range above that of the downstream signal.
 12. The system ofclaim 6 where the ONU attenuates reflections using at least one of: (i)an active amplifier; and (2) a switched filter.
 13. A method comprising:propagating a downstream signal from a head end to each of a pluralityof subscribers through an ONU connected to the head end by an opticalcable and each of the plurality of subscribers by an RF coaxial cable;propagating respective upstream signals from each of the plurality ofsubscribers through the ONU to the head end; and isolating RFreflections in the ONU.
 14. The method of claim 13 including the step ofcanceling reflections at a selective one or more of the head end and acable modem of at least one of the plurality of subscribers.
 15. Themethod of claim 13 where the reflections are isolated by an activeamplifier in the ONU positioned between an RF input to a subscriber anda splitter.
 16. The method of claim 15 including the step of directingupstream signals to a second amplifier using a directional splitter. 17.The method of claim 13 including the step of filtering the respectiveupstream signals from the plurality of subscribers and subsequentlycombining the filtered upstream signals.
 18. The method of claim 13including the step of discarding a range of frequencies in thedownstream signal and uses that range to transmit an upstream signal.19. The method of claim 18 where the range of frequencies is between100-200 MHz.
 20. The method of claim 13 including the step oftransmitting an upstream signal at a frequency range above that of thedownstream signal.
 21. The method of claim 13 including the step oftransmitting an upstream signal in a frequency overlapping with that ofthe downstream signal.